Radio-activity/Chapter 1

by Ernest Rutherford

 

CHAPTER I.

RADIO-ACTIVE SUBSTANCES.

1. Introduction. The close of the old and the beginning of the new century have been marked by a very rapid increase of our knowledge of that most important but comparatively little known subject—the connection between electricity and matter. No study has been more fruitful in surprises to the investigator, both from the remarkable nature of the phenomena exhibited and from the laws controlling them. The more the subject is examined, the more complex must we suppose the constitution of matter in order to explain the remarkable effects observed. While the experimental results have led to the view that the constitution of the atom itself is very complex, at the same time they have confirmed the old theory of the discontinuous or atomic structure of matter. The study of the radio-active substances and of the discharge of electricity through gases has supplied very strong experimental evidence in support of the fundamental ideas of the existing atomic theory. It has also indicated that the atom itself is not the smallest unit of matter, but is a complicated structure made up of a number of smaller bodies.

A great impetus to the study of this subject was initially given by the experiments of Lenard on the cathode rays, and by Röntgen's discovery of the X rays. An examination of the conductivity imparted to a gas by the X rays led to a clear view of the mechanism of the transport of electricity through gases by means of charged ions. This ionization theory of gases has been shown to afford a satisfactory explanation not only of the passage of electricity through flames and vapours, but also of the complicated phenomena observed when a discharge of electricity passes through a vacuum tube. At the same time, a further study of the cathode rays showed that they consisted of a stream of material particles, projected with great velocity, and possessing an apparent mass small compared with that of the hydrogen atom. The connection between the cathode and Röntgen rays and the nature of the latter were also elucidated. Much of this admirable experimental work on the nature of the electric discharge has been done by Professor J. J. Thomson and his students in the Cavendish Laboratory, Cambridge.

An examination of natural substances, in order to see if they gave out dark radiations similar to X rays, led to the discovery of the radio-active bodies which possess the property of spontaneously emitting radiations, invisible to the eye, but readily detected by their action on photographic plates and their power of discharging electrified bodies. A detailed study of the radio-active bodies has revealed many new and surprising phenomena which have thrown much light, not only on the nature of the radiations themselves, but also on the processes occurring in those substances. Notwithstanding the complex nature of the phenomena, the knowledge of the subject has advanced with great rapidity, and a large amount of experimental data has now been accumulated.

In order to explain the phenomena of radio-activity, Rutherford and Soddy have advanced a theory which regards the atoms of the radio-active elements as suffering spontaneous disintegration, and giving rise to a series of radio-active substances which differ in chemical properties from the parent elements. The radiations accompany the breaking-up of the atoms, and afford a comparative measure of the rate at which the disintegration takes place. This theory is found to account in a satisfactory way for all the known facts of radio-activity, and welds a mass of disconnected facts into one homogeneous whole. On this view, the continuous emission of energy from the active bodies is derived from the internal energy inherent in the atom, and does not in any way contradict the law of the conservation of energy. At the same time, however, it indicates that an enormous store of latent energy is resident in the radio-atoms themselves. This store of energy has not been observed previously, on account of the impossibility of breaking up into simpler forms the atoms of the elements by the action of the chemical or physical forces at our command.

On this theory we are witnessing in the radio-active bodies a veritable transformation of matter. This process of disintegration was investigated, not by direct chemical methods, but by means of the property possessed by the radio-active bodies of giving out specific types of radiation. Except in the case of a very active element like radium, the process of disintegration takes place so slowly, that hundreds if not thousands of years would be required before the amount transformed would come within the range of detection of the balance or the spectroscope. In radium, however, the process of disintegration takes place at such a rate that it should be possible within a limited space of time to obtain definite chemical evidence on this question. The recent discovery that helium can be obtained from radium adds strong confirmation to the theory; for helium was indicated as a probable disintegration product of the radio-active elements before this experimental evidence was forthcoming. Several products of the transformation of the radio-active bodies have already been examined, and the further study of these substances promises to open up new and important fields of chemical enquiry.

In this book the experimental facts of radio-activity and the connection between them are interpreted on the disintegration theory. Many of the phenomena observed can be investigated in a quantitative manner, and prominence has been given to work of this character, for the agreement of any theory with the facts, which it attempts to explain, must ultimately depend upon the results of accurate measurement.

The value of any working theory depends upon the number of experimental facts it serves to correlate, and upon its power of suggesting new lines of work. In these respects the disintegration theory, whether or not it may ultimately be proved to be correct, has already been justified by its results.

 

2. Radio-active Substances. The term "radio-active" is now generally applied to a class of substances, such as uranium, thorium, radium, and their compounds, which possess the property of spontaneously emitting radiations capable of passing through plates of metal and other substances opaque to ordinary light. The characteristic property of these radiations, besides their penetrating power, is their action on a photographic plate and their power of discharging electrified bodies. In addition, a strongly radio-active body like radium is able to cause marked phosphorescence and fluorescence on some substances placed near it. In the above respects the radiations possess properties analogous to Röntgen rays, but it will be shown that, for the major part of the radiations emitted, the resemblance is only superficial.

The most remarkable property of the radio-active bodies is their power of radiating energy spontaneously and continuously at a constant rate, without, as far as is known, the action upon them of any external exciting cause. The phenomena at first sight appear to be in direct contradiction to the law of conservation of energy, since no obvious change with time occurs in the radiating material. The phenomena appear still more remarkable when it is considered that the radio-active bodies must have been steadily radiating energy since the time of their formation in the earth's crust.

Immediately after Röntgen's discovery of the production of X rays, several physicists were led to examine if any natural bodies possessed the property of giving out radiations which could penetrate metals and other substances opaque to light. As the production of X rays seemed to be connected in some way with cathode rays, which cause strong fluorescent and phosphorescent effects on various bodies, the substances first examined were those that were phosphorescent when exposed to light. The first observation in this direction was made by Niewenglowski^[1], who found that sulphide of calcium exposed to the sun's rays gave out some rays which were able to pass through black paper. A little later a similar result was recorded by H. Becquerel^[2] for a special calcium sulphide preparation, and by Troost^[3] for a specimen of hexagonal blend. These results were confirmed and extended in a later paper by Arnold^[4]. No satisfactory explanations of these somewhat doubtful results have yet been given, except on the view that the black paper was transparent to some of the light waves. At the same time Le Bon^[5] showed that, by the action of sunlight on certain bodies, a radiation was given out, invisible to the eye, but active with regard to a photographic plate. These results have been the subject of much discussion; but there seems to be little doubt that the effects are due to short ultra-violet light waves, capable of passing through certain substances opaque to ordinary light. These effects, while interesting in themselves, are quite distinct in character from those shown by the radio-active bodies which will now be considered.

 

3. Uranium. The first important discovery in the subject of radio-activity was made in February, 1896, by M. Henri Becquerel^[6], who found that a uranium salt, the double sulphate of uranium and potassium, emitted some rays which gave an impression on a photographic plate enveloped in black paper. These rays were also able to pass through thin plates of metals and other substances opaque to light. The impressions on the plate could not have been due to vapours given off by the substances, since the same effect was produced whether the salt was placed directly on the black paper or on a thin plate of glass lying upon it.

Becquerel found later that all the compounds of uranium as well as the metal itself possessed the same property, and, although the amount of action varied slightly for the different compounds, the effects in all cases were comparable. It was at first natural to suppose that the emission of these rays was in some way connected with the power of phosphorescence, but later observations showed that there was no connection whatever between them. The uranic salts are phosphorescent, while the uranous salts are not. The uranic salts, when exposed to ultra-violet light in the phosphoroscope, give a phosphorescent light lasting about ·01 seconds. When the salts are dissolved in water, the duration is still less. The amount of action on the photographic plate does not depend on the particular compound of uranium employed, but only on the amount of uranium present in the compound. The non-phosphorescent are equally active with the phosphorescent compounds. The amount of radiation given out is unaltered if the active body be kept continuously in darkness. The rays are given out by solutions, and by crystals which have been deposited from solutions in the dark and never exposed to light. This shows that the radiation cannot be due in any way to the gradual emission of energy stored up in the crystal in consequence of exposure to a source of light.

4. The power of giving out penetrating rays thus seems to be a specific property of the element uranium, since it is exhibited by the metal as well as by all its compounds. These radiations from uranium are persistent, and, as far as observations have yet gone, are unchanged, either in intensity or character, with lapse of time. Observations to test the constancy of the radiations for long periods of time have been made by Becquerel. Samples of uranic and uranous salts have been kept in a double box of thick lead, and the whole has been preserved from exposure to light. By a simple arrangement, a photographic plate can be introduced in a definite position above the uranium salts, which are covered with a layer of black paper. The plate is exposed at intervals for 48 hours, and the impression on the plate compared. No perceptible weakening of the radiation has been observed over a period of four years. Mme Curie^[7] has made determinations of the activity of uranium over a space of five years by an electric method described later, but found no appreciable variation during that period.

Since the uranium is thus continuously radiating energy from itself, without any known source of excitation, the question arises whether any known agent is able to affect the rate of its emission. No alteration was observed when the body was exposed to ultra-*violet light or to ultra-red light or to X rays. Becquerel states that the double sulphate of uranium and potassium showed a slight increase of action when exposed to the arc light and to sparks, but he considers that the feeble effect observed was another action superimposed on the constant radiation from uranium. The intensity of the uranium radiation is not affected by a variation of temperature between 200° C. and the temperature of liquid air. This question is discussed in more detail later.

5. In addition to these actions on a photographic plate, Becquerel showed that uranium rays, like Röntgen rays, possess the important property of discharging both positively and negatively electrified bodies. These results were confirmed and extended by Lord Kelvin, Smolan and Beattie^[8]. The writer made a detailed comparison^[9] of the nature of the discharge produced by uranium with that produced by Röntgen rays, and showed that the discharging property of uranium is due to the production of charged ions by the radiation throughout the volume of the gas. The property has been made the basis of a qualitative and quantitative examination of the radiations from all radio-active bodies, and is discussed in detail in chapter II.

The radiations from uranium are thus analogous, as regards their photographic and electrical actions, to Röntgen rays, but, compared with the rays from an ordinary X ray tube, these actions are extremely feeble. While with Röntgen rays a strong impression is produced on a photographic plate in a few minutes or even seconds, several days' exposure to the uranium rays is required to produce a well-marked action, even though the uranium compound, enveloped in black paper, is placed close to the plate. The discharging action, while very easily measurable by suitable methods, is also small compared with that produced by X rays from an ordinary tube.

6. The rays from uranium show no evidence of direct reflection, refraction, or polarization^[10]. While there is no direct reflection of the rays, there is apparently a diffuse reflection produced where the rays strike a solid obstacle. This is due in reality to a secondary radiation set up when the primary rays impinge upon matter. The presence of this secondary radiation at first gave rise to the erroneous view that the rays could be reflected and refracted like ordinary light. The absence of reflection, refraction, or polarization in the penetrating rays from uranium necessarily follows in the light of our present knowledge of the rays. It is now known that the uranium rays, mainly responsible for the photographic action, are deviable by a magnetic field, and are similar in all respects to cathode rays, i.e. the rays are composed of small particles projected at great velocities. The absence of the ordinary properties of transverse light waves is thus to be expected.

7. The rays from uranium are complex in character, and, in addition to the penetrating deviable rays, there is also given off a radiation very readily absorbed by passing through thin layers of metal foil, or by traversing a few centimetres of air. The photographic action due to these rays is very feeble in comparison with that of the penetrating rays, although the discharge of electrified bodies is mainly caused by them. Besides these two types of rays, some rays are emitted which are of an extremely penetrating character and are non-deviable by a magnetic field. These rays are difficult to detect photographically, but can readily be examined by the electric method.

8. The question naturally arose whether the property of spontaneously giving out penetrating radiations was confined to uranium and its compounds, or whether it was exhibited to any appreciable extent by other substances.

By the electrical method, with an electrometer of ordinary sensitiveness, any body which possesses an activity of the order of 1/100 of that of uranium can be detected. With an electroscope of special construction, such as has been designed by C. T. R. Wilson for his experiments on the natural ionization of air, a substance of activity 1/10000 and probably 1/100000 of that of uranium can be detected.

If an active body like uranium be mixed with an inactive body, the resulting activity in the mixture is generally considerably less than that due to the active substance alone. This is due to the absorption of the radiation by the inactive matter present. The amount of decrease largely depends on the thickness of the layer from which the activity is determined.

Mme Curie made a detailed examination by the electrical method of the great majority of known substances, including the very rare elements, to see if they possessed any activity. In cases where it was possible, several compounds of the elements were examined. With the exception of thorium and phosphorus, none of the other substances possessed an activity even of the order of 1/100 of uranium.

The ionization of the gas by phosphorus does not, however, seem to be due to a penetrating radiation like that found in the case of uranium, but rather to a chemical action taking place at its surface. The compounds of phosphorus do not show any activity, and in this respect differ from uranium and the other active bodies.

Le Bon^[11] has also observed that quinine sulphate, if heated and then allowed to cool, possesses for a short time the property of discharging both positively and negatively electrified bodies. It is necessary, however, to draw a sharp line of distinction between phenomena of this kind and those exhibited by the naturally radio-active bodies. While both, under special conditions, possess the property of ionizing the gas, the laws controlling the phenomena are quite distinct in the two cases. For example, only one compound of quinine shows the property, and that compound only when it has been subjected to a preliminary heating. The action of phosphorus depends on the nature of the gas, and varies with temperature. On the other hand, the activity of the naturally radio-active bodies is spontaneous and permanent. It is exhibited by all compounds, and is not, as far as is yet known, altered by change in the chemical or physical conditions.

9. The discharging and photographic action alone cannot be taken as a criterion as to whether a substance is radio-active or not. It is necessary in addition to examine the radiations, and to test whether the actions take place through appreciable thicknesses of all kinds of matter opaque to ordinary light. For example, a body giving out short waves of ultra-violet light can be made to behave in many respects like a radio-active body. As Lenard^[12] has shown, short waves of ultra-violet light will ionize the gas in their path, and will be absorbed rapidly in the gas. They will produce strong photographic action, and may pass through some substances opaque to ordinary light. The similarity to a radio-active body is thus fairly complete as regards these properties. On the other hand, the emission of these light waves, unlike that of the radiations from an active body, will depend largely on the molecular state of the compound, or on temperature and other physical conditions. But the great point of distinction lies in the nature of the radiations from the bodies in question. In one case the radiations behave as transverse waves, obeying the usual laws of light waves, while in the case of a naturally active body, they consist for the most part of a continuous flight of material particles projected from the substance with great velocity. Before any substance can be called "radio-active" in the sense in which the term is used to describe the properties of the natural radio-active elements, it is thus necessary to make a close examination of its radiation; for it is unadvisable to extend the use of the term "radio-active" to substances which do not possess the characteristic radiating properties of the radio-active elements which we have described, and the active products which can be obtained from them. Some of the pseudo-active bodies will however be considered later in chapter IX.

10. Thorium. In the course of an examination of a large number of substances, Schmidt^[13] found that thorium, its compounds, and the minerals containing thorium, possessed properties similar to those of uranium. The same discovery was made independently by Mme Curie^[14]. The rays from thorium compounds, like those from uranium, possess the properties of discharging electrified bodies and acting on a photographic plate. Under the same conditions the discharging action of the rays is about equal in amount to that of uranium, but the photographic effect is distinctly weaker.

The radiations from thorium are more complicated than those from uranium. It was early observed by several experimenters that the radiation from thorium compounds, especially the oxide, when tested by the electrified method, was very variable and uncertain. A detailed investigation of the radiations from thorium under various conditions was made by Owens^[15]. He showed that thorium oxide, especially in thick layers, was able to produce conductivity in the gas when covered with a large thickness of paper, and that the amount of this conductivity could be greatly varied by blowing a current of air over the gas. In the course of an examination^[16] of this action of the air current, the writer showed that thorium compounds gave out a material emanation made up of very small particles themselves radio-active. The emanation behaves like a radio-active gas; it diffuses rapidly through porous substances like paper, and is carried away by a current of air. The evidence of the existence of the emanation and its properties, is considered in detail later in chapter VIII. In addition to giving out an emanation, thorium behaves like uranium in emitting three types of radiation, each of which is similar in properties to the corresponding radiation from uranium.

11. Radio-active minerals. Mme Curie has examined the radio-activity of a large number of minerals containing uranium and thorium. The electrical method was used, and the current measured between two parallel plates 8 cms. in diameter and 3 cms. apart, when one plate was covered with a uniform layer of the active matter. The following numbers give the order of the saturation current obtained in amperes.

                                          i
Pitchblende from Johanngeorgenstadt 8·3 × 10^{-11}
     " Joachimsthal 7·0 "
     " Pzibran 6·5 "
     " Cornwall 1·6 "
Clevite 1·4 "
Chalcolite 5·2 "
Autunite 2·7 "
Thorite from 0·3 to 1·4 "
Orangite 2·0 "
Monazite 0·5 "
Xenotine 0·03 "
Aeschynite 0·7 "
Fergusonite 0·4 "
Samarskite 1·1 "
Niobite 0·3 "
Carnotite 6·2 "

Some activity is to be expected in these minerals, since they all contain either thorium or uranium or a mixture of both. An examination of the action of the uranium compounds with the same apparatus and under the same conditions led to the following results:

                                           i
Uranium (containing a little carbon) 2·3 × 10^{-11} amperes
Black oxide of uranium 2·6 "
Green " " 1·8 "
Acid uranic hydrate 0·6 "
Uranate of sodium 1·2 "
Uranate of potassium 1·2 "
Uranate of ammonia 1·3 "
Uranous sulphate 0·7 "
Sulphate of uranium and potassium 0·7 "
Acetate 0·7 "
Phosphate of copper and uranium 0·9 "
Oxysulphide of uranium 1·2 "

The interesting point in connection with these results is that some specimens of pitchblende have four times the activity of the metal uranium; chalcolite, the crystallized phosphate of copper and uranium, is twice as active as uranium; and autunite, a phosphate of calcium and uranium, is as active as uranium. From the previous considerations, none of the substances should have shown as much activity as uranium or thorium. In order to be sure that the large activity was not due to the particular chemical combination, Mme Curie prepared chalcolite artificially, starting with pure products. This artificial chalcolite had the activity to be expected from its composition, viz. about 0·4 of the activity of the uranium. The natural mineral chalcolite is thus five times as active as the artificial mineral.

It thus seemed probable that the large activity of some of these minerals, compared with uranium and thorium, was due to the presence of small quantities of some very active substance, which was different from the known bodies thorium and uranium.

This supposition was completely verified by the work of M. and Mme Curie, who were able to separate from pitchblende by purely chemical methods two active bodies, one of which in the pure state is over a million times more active than the metal uranium.

This important discovery was due entirely to the property of radio-activity possessed by the new bodies. The only guide in their separation was the activity of the products obtained. In this respect the discovery of these bodies is quite analogous to the discovery of rare elements by the methods of spectrum analysis. The method employed in the separation consisted in examining the relative activity of the products after chemical treatment. In this way it was seen whether the radio-activity was confined to one or another of the products, or divided between both, and in what ratio such division occurred.

The activity of the specimens thus served as a basis of rough qualitative and quantitative analysis, analogous in some respects to the indication of the spectroscope. To obtain comparative data it was necessary to test all the products in the dry state. The chief difficulty lay in the fact that pitchblende is a very complex mineral, and contains in varying quantities nearly all the known metals.

12. Radium. The analysis of pitchblende by chemical methods, using the procedure sketched above, led to the discovery of two very active bodies, polonium and radium. The name polonium was given to the first substance discovered by Mme Curie in honour of the country of her birth. The name radium was a very happy inspiration of the discoverers, for this substance in the pure state possesses the property of radio-activity to an astonishing degree.

Radium is extracted from pitchblende by the process used to separate barium, to which radium is very closely allied in chemical properties^[17]. After the removal of other substances, the radium remains behind mixed with barium. It can, however, be partially separated from the latter by the difference in solubility of the chlorides in water, alcohol, or hydrochloric acid. The chloride of radium is less soluble than that of barium, and can be separated from it by the method of fractional crystallization. After a large number of precipitations, the radium can be freed almost completely from the barium.

Both polonium and radium exist in infinitesimal quantities in pitchblende. In order to obtain a few decigrammes of very active radium, it is necessary to use several tons of pitchblende, or the residues obtained from the treatment of uranium minerals. It is thus obvious that the expense and labour involved in preparation of a minute quantity of radium are very great.

M. and Mme Curie were indebted for their first working material to the Austrian government, who generously presented them with a ton of the treated residue of uranium materials from the State manufactory of Joachimsthal in Bohemia. With the assistance of the Academy of Science and other societies in France, funds were given to carry out the laborious work of separation. Later the Curies were presented with a ton of residues from the treatment of pitchblende by the Société Centrale de Produits Chimiques of Paris. The generous assistance afforded in this important work is a welcome sign of the active interest taken in these countries in the furthering of purely scientific research.

The rough concentration and separation of the residues was performed in the chemical works, and there followed a large amount of labour in purification and concentration. In this manner, the Curies were able to obtain a small quantity of radium which was enormously active compared with uranium. No definite results have yet been given on the activity of pure radium, but the Curies estimate that it is about one million times that of uranium, and may possibly be still higher. The difficulty of making a numerical estimate for such an intensely active body is very great. In the electric method, the activities are compared by noting the relative strength of the maximum or saturation current between two parallel plates, on one of which the active substance is spread. On account of the intense ionization of the gas between the plates, it is not possible to reach the saturation current unless very high voltages are applied. Approximate comparisons can be made by the use of metal screens to cut down the intensity of the radiations, if the proportion of the radiation transmitted by such a screen has been determined by direct experiment on impure material of easily measurable activity. The value of the activity of radium compared with that of uranium will however vary to some extent according to which of the three types of rays is taken as a basis of comparison.

It is thus difficult to control the final stages of the purification of radium by measurements of its activity alone. Moreover the activity of radium immediately after its preparation is only about one-fourth of its final value; it gradually rises to a maximum after the radium salt has been kept in the dry state for about a month. For control experiments in purification, it is advisable to measure the initial rather than the final activity.

Mme Curie has utilized the coloration of the crystals of radiferous barium as a means of controlling the final process of purification. The crystals of salts of radium and barium deposited from acid solutions are indistinguishable by the eye. The crystals of radiferous barium are at first colourless, but, in the course of a few hours, become yellow, passing to orange and sometimes to a beautiful rose colour. The rapidity of this coloration depends on the amount of barium present. Pure radium crystals do not colour, or at any rate not as rapidly as those containing barium. The coloration is a maximum for a definite proportion of radium, and this fact can be utilized as a means of testing the amount of barium present. When the crystals are dissolved in water the coloration disappears.

Giesel^[18] has observed that pure radium bromide gives a beautiful carmine colour to the Bunsen flame. If barium be present in any quantity, only the green colour due to barium is observed, and a spectroscopic examination shows only the barium lines. This carmine coloration of the Bunsen flame is a good indication of the purity of the radium.

Since the preliminary announcement of the discovery of radium, Giesel^[19] has devoted a great deal of attention to the separation of radium, polonium and other active bodies from pitchblende. He was indebted for his working material to the firm of P. de Haen, of Hanover, who presented him with a ton of pitchblende residues. Using the method of fractional crystallization of the bromide instead of the chloride, he has been able to prepare considerable quantities of pure radium. By this means the labour of final purification of radium has been much reduced. He states that six or eight crystallizations with the bromide are sufficient to free the radium almost completely from the barium.

13. Spectrum of radium. It was of great importance to settle as soon as possible whether radium was in reality modified barium or a new element with a definite spectrum. For this purpose the Curies prepared some specimens of radium chloride, and submitted them for examination of their spectrum to Demarçay, an authority on that subject. The first specimen of radium chloride examined by Demarçay^[20] was not very active, but showed, besides the lines due to barium, a very strong new line in the ultra-violet. In another sample of greater activity, the line was still stronger and others also appeared, while the intensity of the new lines was comparable with those present due to barium. With a still more active specimen which was probably nearly pure, only three strong lines of barium appeared, while the new spectrum was very bright. The following table shows the wave-length of the new lines observed for radium. The wave lengths are expressed in Ångström units and the intensity of each ray is denoted by a number, the ray of maximum intensity being 16.

Wave length Intensity Wave length Intensity
  4826·3 10 4600·3 3
  4726·9 5 4533·5 9
  4699·6 3 4436·1 6
  4692·1 7 4340·6 12
  4683·0 14 3814·7 16
  4641·9 4 3649·6 12

The lines are all sharply defined, and three or four of them have an intensity comparable with any known lines of other substances. There are also present in the spectrum two strong nebulous bands. In the visible part of the spectrum, which has not been photographed, the only noticeable ray has a wave length 5665, which is, however, very feeble compared with that of wave length 4826·3. The general aspect of the spectrum is similar to that of the alkaline earths; it is known that these metals have strong lines accompanied by nebulous bands.

The principal line due to radium can be distinguished in impure radium of activity 50 times that of uranium. By the electrical method it is easy to distinguish the presence of radium in a body which has an activity only 1/100 of uranium. With a more sensitive electrometer 1/10000 of the activity of uranium could be observed. For the detection of radium, the examination of the radio-activity is thus a process nearly a million times more sensitive than spectrum analysis.

Later observations on the spectrum of radium have been made by Runge^[21], Exner and Haschek^[22], with specimens of radium prepared by Giesel. Crookes^[23] has photographed the spectrum of radium in the ultra-violet, while Runge and Precht^[24], using a highly purified sample of radium, observed a number of new lines in the spark spectrum. It has been mentioned already that the bromide of radium gives a characteristic pure carmine-red coloration to the Bunsen flame. The flame spectrum shows two broad bright bands in the orange-red, not observed in Demarçay's spectrum. In addition there is a line in the blue-green and two feeble lines in the violet.

14. Atomic weight of radium. Mme Curie has made successive determinations of the atomic weight of the new element with specimens of steadily increasing purity. In the first observation the radium was largely mixed with barium, and the atomic weight obtained was the same as that of barium, 137·5. In successive observations with specimens of increasing purity the atomic weights of the mixture were 146 and 175. The final value obtained recently was 225, which may be taken as the atomic weight of radium on the assumption that it is divalent.

In these experiments about 0·1 gram of pure radium chloride was obtained by successive fractionations. The difficulty involved in preparing a quantity of pure radium chloride large enough to test the atomic weight may be gauged from the fact that only a few centigrams of fairly pure radium, or a few decigrams of less concentrated material, are obtained from the treatment of about 2 tons of the mineral from which it is derived.

Runge and Precht^[25] have examined the spectrum of radium in a magnetic field, and have shown the existence of series analogous to those observed for calcium, barium, and strontium. These series are connected with the atomic weights of the elements in question, and Runge and Precht have calculated by these means that the atomic weight of radium should be 258—a number considerably greater than the number 225 obtained by Mme Curie by means of chemical analysis. Marshall Watts^[26], on the other hand, using another relation between the lines of the spectrum, deduced the value obtained by Mme Curie. Runge^[27] has criticised the method of deduction employed by Marshall Watts on the ground that the lines used for comparison in the different spectra were not homologous. Considering that the number found by Mme Curie agrees with that required by the periodic system, it is advisable in the present state of our knowledge to accept the experimental number rather than the one deduced by Runge and Precht from spectroscopic evidence.

There is no doubt that radium is a new element possessing remarkable physical properties. The detection and separation of this substance, existing in such minute proportions in pitchblende, has been due entirely to the characteristic property we are considering, and is the first notable triumph of the study of radio-activity. As we shall see later, the property of radio-activity can be used, not only as a means of chemical research, but also as an extraordinarily delicate method of detecting chemical changes of a very special kind.

15. Radiations from radium. On account of its enormous activity, the radiations from radium are very intense: a screen of zinc sulphide, brought near a few centigrams of radium bromide, is lighted up quite brightly in a dark room, while brilliant fluorescence is produced on a screen of platino-barium cyanide. An electroscope brought near the radium salt is discharged almost instantly, while a photographic plate is immediately affected. At a distance of one metre, a day's exposure to the radium rays would produce a strong impression. The radiations from radium are analogous to those of uranium, and consist of three types of rays: easily absorbed, penetrating, and very penetrating. Radium also gives rise to an emanation similar to that of thorium, but with a very much slower rate of decay. The radium emanation retains its activity for several weeks, while that of thorium lasts only a few minutes. The emanation obtained from a few centigrams of radium illuminates a screen of zinc sulphide with great brilliancy. The very penetrating rays of radium are able to light up an X ray screen in a dark room, after passage through several centimetres of lead and several inches of iron.

As in the case of uranium or thorium, the photographic action is mainly due to the penetrating or cathodic rays. The radiographs obtained with radium are very similar to those obtained with X rays, but lack the sharpness and detail of the latter. The rays are unequally absorbed by different kinds of matter, the absorption varying approximately as the density. In photographs of the hand the bones do not stand out as in X ray photographs.

Curie and Laborde have shown that the compounds of radium possess the remarkable property of always keeping their temperature several degrees above the temperature of the surrounding air. Each gram of radium radiates an amount of energy corresponding to 100 gram-calories per hour. This and other properties of radium are discussed in detail in chapters V and XII.

16. Compounds of radium. When first prepared in the solid state, all the salts of radium—the chloride, bromide, acetate, sulphate, and carbonate—are very similar in appearance to the corresponding salts of barium, but in time they gradually become coloured. In chemical properties the salts of radium are practically the same as those of barium, with the exception that the chloride and bromide of radium are less soluble in water than the corresponding salts of barium. All the salts of radium are naturally phosphorescent. The phosphorescence of impure radium preparations is in some cases very marked.

All the radium salts possess the property of causing rapid colorations of the glass vessel which contains them. For feebly active material the colour is usually violet, for more active material a yellowish-brown, and finally black.

17. Actinium. The discovery of radium in pitchblende gave a great impetus to the chemical examination of uranium residues, and a systematic search early led to the detection of several new radio-active bodies. Although these show distinctive radio-active properties, so far none of them have been purified sufficiently to give a definite spectrum as in the case of radium. One of the most interesting and important of these substances was discovered by Debierne^[28] while working up the uranium residues, obtained by M. and Mme Curie from the Austrian government, and was called by him actinium. This active substance is precipitated with the iron group, and appears to be very closely allied in chemical properties to thorium, though it is many thousand times more active. It is very difficult to separate from thorium and the rare earths. Debierne has made use of the following methods for partial separation:

(1) Precipitation in hot solutions, slightly acidulated with hydrochloric acid, by excess of hyposulphite of soda. The active matter is present almost entirely in the precipitate.

(2) Action of hydrofluoric acid upon the hydrates freshly precipitated, and held in suspension in water. The portion dissolved is only slightly active. By this method titanium may be separated.

(3) Precipitation of neutral nitrate solutions by oxygenated water. The precipitate carries down the active body.

(4) Precipitation of insoluble sulphates. If barium sulphate, for example, is precipitated in the solution containing the active body, the barium carries down the active matter. The thorium and actinium are freed from the barium by conversion of the sulphate into the chloride and precipitation by ammonia.

In this way Debierne has obtained a substance comparable in activity with radium. The separation, which is difficult and laborious, has not yet been carried far enough to bring out any new lines in the spectrum.

18. After the initial announcement of the discovery of actinium, several years elapsed before any definite results upon it were published by Debierne. In the meantime, Giesel^[29] had independently obtained a radio-active substance from pitchblende which seemed similar in many respects to the actinium of Debierne. The active substance belongs to the group of cerium earths and is precipitated with them. By a succession of chemical operations, the active substance is separated mixed with lanthanum. While intensely active in comparison with thorium, the new active substance closely resembles it in radio-active properties, although, from the method of separation thorium cannot be present except in minute quantity. Giesel early observed that the substance gave off a radio-active emanation. On account of the intensity of the emanation it emits, he termed it the "emanating substance." Recently this name has been changed to "emanium," and under this title preparations of the active substance prepared by Giesel have been placed on the market.

Giesel found that the activity of this substance was permanent and seemed to increase during the six months' interval after separation. In this respect it is similar to radium compounds, for the activity of radium, measured by the electric method, increases in the course of a month's interval to four times its initial value at separation.

There can be no doubt that the "actinium" of Debierne and the "emanium" of Giesel contain the same radio-active constituent, for recent work^[30] has shown that they exhibit identical radio-active properties. Each gives out easily absorbed and penetrating rays, and emits a characteristic emanation of which the rate of decay is the same for both substances. The rate of decay of the emanation is the simplest method of distinguishing actinium from thorium, which it resembles so closely in radio-active as well as in chemical properties. The emanation of actinium loses its radiating power far more rapidly than that of thorium, the time taken for the activity to fall to half value being in the two cases 3·7 seconds and 52 seconds respectively.

The rapid and continuous emission of this short-lived emanation is the most striking radio-active property possessed by actinium. In still air, the radio-active effects of this emanation are confined to a distance of a few centimetres from the active material, as it is only able to diffuse a short distance through the air before losing its radiating power. With very active preparations of actinium, the material appears to be surrounded by a luminous haze produced by the emanation. The radiations produce strong luminosity in some substances, for example, zinc sulphide, willemite and platino-*cyanide of barium. The luminosity is especially marked on screens of zinc sulphide. Much of this effect is due to the emanation, for, on gently blowing a current of air over the substance, the luminosity is displaced at once in the direction of the current. With a zinc sulphide screen, actinium shows the phenomena of "scintillations" to an even more marked degree than radium itself.

The preparations of emanium are in some cases luminous, and a spectroscopic examination of this light has shown a number of bright lines^[31].

The distinctive character of the emanation of actinium, as well as of the other radio-active products to which it gives rise, coupled with the permanence of its activity, renders it very probable that actinium will prove to be a new radio-active element of very great activity. Although very active preparations of actinium have been obtained, it has not yet been found possible to free it from impurities. Consequently, no definite observations have been made on its chemical properties, and no new spectrum lines have been observed.

A more complete discussion of the radio-active and other properties of actinium is given in later chapters.

19. Polonium. Polonium was the first of the active substances obtained from pitchblende. It has been investigated in detail by its discoverer Mme Curie^[32]. The pitchblende was dissolved in acid and sulphuretted hydrogen added. The precipitated sulphides contained an active substance, which, after separation of impurities, was found associated with bismuth. This active substance, which has been named polonium, is so closely allied in chemical properties to bismuth that it has so far been found impossible to effect a complete separation. Partial separation of polonium can be made by successive fractionations based on one of the following modes of procedure:

(1) Sublimation in a vacuum. The active sulphide is more volatile than that of bismuth. It is deposited as a black substance at those parts of the tube, where the temperature is between 250 and 300° C. In this way polonium of activity 700 times that of uranium was obtained.

(2) Precipitation of nitric acid solutions by water. The precipitated sub-nitrate is much more active than the part that remains in solution.

(3) Precipitation by sulphuretted hydrogen in a very acid hydrochloric acid solution. The precipitated sulphides are much more active than the salt which remains in solution.

For concentration of the active substance Mme Curie^[33] has made use of method (2). The process is, however, very slow and tedious, and is made still more complicated by the tendency to form precipitates insoluble either in strong or weak acids. After a large number of fractionations, a small quantity of matter was obtained, enormously active compared with uranium. On examination of the substance spectroscopically, only the bismuth lines were observed. A spectroscopic examination of the active bismuth by Demarçay and by Runge and Exner has led to the discovery of no new lines. On the other hand Sir William Crookes^[34] states that he found one new line in the ultra-violet, while Berndt^[35], working with polonium of activity 300, observed a large number of new lines in the ultra-violet. These results await further confirmation.

The polonium prepared by Mme Curie differs from the other radio-active bodies in several particulars. In the first place the radiations include only very easily absorbable rays. The two penetrating types of radiation given out by uranium, thorium, and radium are absent. In the second place the activity does not remain constant, but diminishes continuously with the time. Mme Curie states that different preparations of polonium had somewhat different rates of decay. In some cases, the activity fell to half value in about six months, and in others, about half value in eleven months.

20. The gradual diminution of the activity of polonium with time seemed at first sight to differentiate it from such substances as uranium and radium, the activity of which appeared fairly permanent. This difference in behaviour is, however, one of degree rather than of kind. We shall show later that there is present in pitchblende a number of radio-active substances, the activity of which is not permanent. The time taken for these bodies to lose half of their activity varies in different cases from a few seconds to several hundreds of years. In fact, this gradual loss of activity is an essential feature of our theory of regarding the phenomena of radio-activity. No radio-active substance, left to itself, can continue to radiate indefinitely; it must ultimately lose its activity. In the case of bodies like uranium and radium, the loss of activity is so slow that no sensible alteration has been observed over a period of several years, but it can be deduced theoretically that the activity of radium will eventually decrease to half value in a period of about 1000 years, while in the case of a feebly radio-active substance like uranium, more than a 100 million years must elapse before the diminution of the activity becomes appreciable.

It may be of interest here to consider briefly the suggestions advanced at various times to account for the temporary character of the activity of polonium. Its association with bismuth led to the view that polonium was not a new active substance, but merely radio-active bismuth, that is, bismuth which in some way had been made active by admixture with radio-active bodies. It was known that a body placed in the vicinity of thorium or radium became temporarily active. The same action was supposed to take place when inactive matter was in solution with active matter. The non-active matter was supposed to acquire activity by "induction," as it was called, in consequence of its intimate contact with the active material.

There is no proof, however, that such is the case. The evidence points rather to the conclusion that the activity is due, not to an alteration of the inactive body itself, but to an admixture with it of a very small quantity of intensely active matter. This active matter is present in pitchblende and is separated with the bismuth but differs from it in chemical properties.

The subject cannot be considered with advantage at this stage, but will be discussed later in detail in chapter XI. It will there be shown that polonium, that is, the radio-active constituent mixed with the bismuth, is a distinct chemical substance, which is allied in chemical properties to bismuth, but possesses some distinct analytical properties which allow of a partial separation from it.

The polonium, if obtained in a pure state, should initially be several hundred times as active as pure radium. This activity, however, is not permanent; it decays with the time, falling to half value in about six months.

The absence of any new lines in the spectrum of radio-active bismuth is to be expected, for, even in the most active bismuth prepared, the active matter exists in a very small proportion.

21. The discussion of the nature of polonium was renewed by the discovery of Marckwald^[36] that a substance similar to polonium can be separated from pitchblende; the activity of this substance, he stated, did not decay appreciably with the time. The method of separation from the bismuth chloride solution, obtained from uranium residues, was very simple. A rod of bismuth or antimony, dipped in the active solution, rapidly became coated with a black deposit which was intensely active. This process was continued until the whole of the activity was removed from the solution. The active deposit gave out only easily absorbed rays, and in that respect resembled the polonium of Mme Curie.

The active substance was found to consist mainly of tellurium, and for this reason Marckwald gave it the name of radio-tellurium. In later work, however, Marckwald^[37] has shown that the active constituent has no connection with tellurium, but can always be separated completely from it by a simple chemical process.

In order to obtain a large amount of the active substance, 2000 kilos. of pitchblende were worked up. This yielded 6 kilos. of bismuth oxychloride, and from this was separated 1·5 grams of radio-tellurium. The tellurium present was precipitated from a hydrochloric acid solution by hydrazine hydrochloride. The precipitated tellurium still showed some activity, but this was removed by repeating the process. The active matter then remained in the filtrate, and, after evaporation, the addition of a few drops of stannous chloride caused a small quantity of a dark precipitate which was intensely active. This was collected on a filter and weighed only 4 milligrams.

When plates of copper, tin or bismuth were dipped into an hydrochloric acid solution of this active substance, the plates were found to be covered with a very finely divided deposit. These plates were intensely active, and produced marked photographic and phosphorescent action. As an illustration of the enormous activity of this deposit, Marckwald stated that a precipitate of 1/100 milligram on a copper plate, 4 square centimetres in area, illuminated a zinc sulphide screen so brightly that it could be seen by an audience of several hundred people.

The active substance of Marckwald is very closely allied in chemical and radio-active properties to the polonium of Mme Curie. Both active substances are separated with bismuth and both give out only easily absorbed rays. The penetrating rays, such as are given out by uranium, radium or thorium, are completely absent.

There has been a considerable amount of discussion as to whether the active substance obtained by Marckwald is identical with that present in the polonium of Mme Curie. Marckwald stated that his active substance did not sensibly diminish in activity in the course of six months, but it is doubtful whether the method of measurement used was sufficiently precise.

The writer has found that radio-tellurium of moderate activity, prepared after Marckwald's method and sold by Dr Sthamer of Hamburg, undoubtedly loses its activity with time. The radio-tellurium is obtained in the form of a thin radio-active deposit on a polished bismuth rod or plate. A bismuth rod was found to have lost half its activity in about 150 days, and a similar result has been recorded by other observers.

The two substances are thus similar in both radio-active and chemical properties, and there can be no reasonable doubt that the active constituent present in each case is the same. The evidence is discussed in detail in chapter XI and it will there be shown that the active substance present in the radio-tellurium of Marckwald is a slow transformation product of radium.

22. Radio-active lead. Several observers early noticed that the lead separated from pitchblende showed strong radio-active properties, but considerable difference of opinion was expressed in regard to the permanence of its activity. Elster and Geitel^[38] found that lead sulphate obtained from pitchblende was very active, but they considered that the activity was probably due to an admixture of radium or polonium with the lead, and, by suitable chemical treatment, the lead sulphate was obtained in an inactive state. Giesel^[39] also separated some radio-active lead but found that its activity diminished with the time. On the other hand, Hofmann and Strauss^[40] obtained lead from pitchblende whose activity seemed fairly permanent. They state that the radio-active lead resembled ordinary lead in most of its reactions, but showed differences in the behaviour of the sulphide and sulphate. The sulphate was found to be strongly phosphorescent. These results of Hofmann and Strauss were subjected at the time of their publication to considerable criticism, and there is no doubt that the lead itself is not radio-active but contains a small quantity of radio-active matter which is separated with it. In later work^[41], it has been shown that radio-lead contains several radio-active constituents which can be removed temporarily from it by suitable chemical methods.

There can be no doubt that the lead separated from pitchblende by certain methods does show considerable activity and that this activity is fairly permanent. The radio-active changes occurring in radio-lead are complicated and cannot be discussed with advantage at this stage, but will be considered in detail in chapter XI. It will there be shown that the primary constituent present in lead is a slow transformation product of radium. This substance then slowly changes into the active constituent present in polonium, which gives out only easily absorbed rays.

This polonium can be separated temporarily from the lead by suitable chemical methods, but the radio-lead still continues to produce polonium, so that a fresh supply may be obtained from it, provided an interval of several months is allowed to elapse.

It will be calculated later that in all probability the radio-lead would lose half of its activity in an interval of 40 years.

The constituent present in radio-lead has not yet been separated, but it will be shown that, in the pure state, it should have an activity considerably greater than that of radium itself. Sufficient attention has not yet been paid to this substance, for, separated in a pure state, it should be as useful scientifically as radium. In addition, since it is the parent of polonium, it should be possible to obtain from it at any time a supply of very active polonium, in the same way that a supply of the radium emanation can be obtained at intervals from radium.

Hofmann and Strauss have observed a peculiar action of the cathode rays on the active lead sulphate separated by them. They state that the activity diminishes with time, but is recovered by exposure of the lead for a short time to the action of cathode rays. No such action is shown by the active lead sulphide. This effect is due most probably to the action of the cathode rays in causing a strong phosphorescence of the lead sulphate and has nothing to do with the radio-activity proper of the substance.

23. Is thorium a radio-active element? The similarity of the chemical properties of actinium and thorium has led to the suggestion at different times that the activity of thorium is not due to thorium itself, but to the presence of a slight trace of actinium. In view of the difference in the rate of decay of the emanations of thorium and actinium, this position is not tenable. If the activity of thorium were due to actinium, the two emanations, as well as the other products obtained from these substances, should have identical rates of decay. Since there is not the slightest evidence that the rate of decay of activity of the various products can be altered by chemical or physical agencies, we may conclude with confidence that whatever radio-active substance is responsible for the activity of thorium, it certainly is not actinium. This difference in the rate of decay of the active products is of far more weight in deciding the question whether two bodies contain the same radio-active constituent than differences in chemical behaviour, for it is quite probable that the active material in each case may exist only in minute quantity in the matter under examination, and, under such conditions, a direct chemical examination in the first place is of little value.

Recent work of Hofmann and Zerban and of Baskerville, however, certainly tends to show that the element thorium is itself non-radio-active, and that the radio-activity observed in ordinary thorium compounds is due to the admixture with it of an unknown radio-active element. Hofmann and Zerban^[42] made a systematic examination of the radio-activity of thorium obtained from different mineral sources. They found generally that thorium, obtained from minerals containing a large percentage of uranium, were more active than those obtained from minerals nearly free from uranium. This indicates that the radio-activity observed in thorium may possibly be due to a transformation product of uranium which is closely allied chemically to thorium and is always separated with it. A small quantity of thorium obtained from the mineral gadolinite was found by Hofmann to be almost inactive, whether tested by the electric or by the photographic method. Later Baskerville and Zerban^[43] found that thorium obtained from a Brazilian mineral was practically devoid of activity.

In this connection the recent work of Baskerville on the complexity of ordinary thorium is of interest. By special chemical methods, he succeeded in separating two new and distinct substances from thorium, which he has named carolinium and berzelium. Both of these substances are strongly radio-active, and it thus seems probable that the active constituent observed in ordinary thorium may be due to one of these elements.

If, as we have suggested, thorium itself is not active, it is certainly a matter of surprise that ordinary commercial thorium and the purest chemical preparations show about the same activity. Such a result indicates that the methods of purification have not removed any of the radio-active constituent originally present.

Whatever the radio-active constituent in thorium may ultimately prove to be, it is undoubtedly not radium nor actinium nor any of the known radio-active substances. In later chapters, the radio-activity of thorium will, for simplicity, be discussed on the assumption that thorium is itself a radio-active element. The analysis of the changes which occur will thus not refer to thorium itself but to the primary radio-active substance usually found associated with it. The conclusions to be drawn from an examination of the radio-active processes are for the most part independent of whether thorium is itself radio-active or whether the radio-activity is due to an unknown element. If thorium is not radio-active itself, it is not possible to draw any conclusions upon the question of the duration of the primary radio-activity associated with it. Such a deduction cannot be made until the quantity of the radio-active element present in thorium has been definitely determined.

24. If elements heavier than uranium exist, it is probable that they will be radio-active. The extreme delicacy of radio-activity as a means of chemical analysis would enable such elements to be recognized even if present in infinitesimal quantities. It is probable that considerably more than the three or four radio-elements at present recognized exist in minute quantity, and that the number at present known will be augmented in the future. In the first stage of the search, a purely chemical examination is of little value, for it is not probable that the new element should exist in sufficient quantity to be detected by chemical or spectroscopic analysis. The main criteria of importance are the existence or absence of distinctive radiations or emanations, and the permanence of the radio-activity. The discovery of a radio-active emanation with a rate of decay different from those already known would afford strong evidence that a new radio-active body was present. The presence of either thorium or radium in matter can very readily be detected by observing the rate of decay of the emanations given out by them. When once the existence of a new radio-element has been inferred by an examination of its radio-active properties, chemical methods of separation can be devised, the radiating or emanating property being used as a guide in qualitative and quantitative analysis.

1. Niewenglowski, C. R. 122, p. 385, 1896.
2. Becquerel, C. R. 122, p. 559, 1896.
3. Troost, C. R. 122, p. 564, 1896.
4. Arnold, Annal. d. Phys. 61, p. 316, 1897.
5. Le Bon, C. R. 122, pp. 188, 233, 386, 462, 1896.
6. Becquerel, C. R. 122, pp. 420, 501, 559, 689, 762, 1086, 1896.
7. Mme Curie, Thèse présentée à la Faculté des Sciences de Paris, 1903.
8. Nature, 56, 1897; Phil. Mag. 43, p. 418, 1897; 45, p. 277, 1898.
9. Rutherford, Phil. Mag. Jan. 1899.
10. Ibid.
11. Le Bon, C. R. 130, p. 891, 1900.
12. Lenard, Annal. d. Phys. 1, p. 498; 3, p. 298, 1900.
13. Schmidt, Annal. d. Phys. 65, p. 141, 1898.
14. Mme Curie, C. R. 126, p. 1101, 1898.
15. Owens, Phil. Mag. Oct. 1899.
16. Rutherford, Phil. Mag. Jan. 1900.
17. M. and Mme Curie and G. Bemont, C. R. 127, p. 1215, 1898.
18. Giesel, Phys. Zeit. 3, No. 24, p. 578, 1902.
19. Giesel, Annal. d. Phys. 69, p. 91, 1890. Ber. d. D. Chem. Ges. p. 3608, 1902.
20. Demarçay, C. R. 127, p. 1218, 1898; 129, p. 716, 1899; 131, p. 258, 1900.
21. Runge, Astrophys. Journal, p. 1, 1900. Annal. d. Phys. No. 10, p. 407, 1903.
22. Exner and Haschek, Wien. Ber. July 4, 1901.
23. Crookes, Proc. Roy. Soc. 72, p. 295, 1904.
24. Runge and Precht, Annal. d. Phys. XIV. 2, p. 418, 1904.
25. Runge and Precht, Phil. Mag. April, 1903.
26. Watts, Phil. Mag. July, 1903; August, 1904.
27. Runge, Phil. Mag. December, 1903.
28. Debierne, C. R. 129, p. 593, 1899; 130, p. 206, 1900.
29. Giesel, Ber. d. D. Chem. Ges. p. 3608, 1902; p. 342, 1903.
30. Debierne, C. R. 139, p. 538, 1904. Miss Brooks, Phil. Mag. Sept. 1904. Giesel, Phys. Zeit. 5, p. 822, 1904. Jahrbuch. d. Radioaktivität, no. 4, p. 345, 1904.
31. Giesel, Ber. d. D. Chem. Ges. 37, p. 1696, 1904; Hartmann, Phys. Zeit. 5, No. 18, p. 570, 1904.
32. Mme Curie, C. R. 127, p. 175, 1898.
33. Mme Curie, Thèse, Paris, 1903.
34. Crookes, Proc. Roy. Soc. May, 1900.
35. Berndt, Phys. Zeit. 2, p. 180, 1900.
36. Marckwald, Phys. Zeit. 4, No. 1 b, p. 51.
37. Marckwald, Ber. d. D. Chem. Ges. p. 2662, No. 12, 1903.
38. Elster and Geitel, Annal. d. Phys. 69, p. 83, 1899.
39. Giesel, Ber. d. D. Chem. Ges. p. 3775, 1901.
40. Hofmann and Strauss, Ber. d. D. Chem. Ges. p. 3035, 1901.
41. Hofmann, Gonder and Wölfl, Annal. d. Phys. No. 13, p. 615, 1904.
42. Hofmann and Zerban, Ber. d. D. Chem. Ges. No. 12, p. 3093, 1903.
43. Baskerville and Zerban, Amer. Chem. Soc. 26, p. 1642, 1904.