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first indicated by Dumas and Boullay, and the view of the constitu tion of the ammonia salts which is generally held in France was the reason why ether was considered the first hydrate of olefiant gas, alcohol as the second hydrate, &c.; in Germany and other countries the water necessary for the constitution of the salts of ammonia with oxygen acids was considered as an integral part of the base; it was assumed that this water forms with the ammonia oxide of ammonium (NH 4 ).,0, and this view in a certain sense smoothed the way for another, according to which the existence of organic oxides, capable of neutralizing acids, appeared very probable, as a necessary complement to the organic acids which chemists had long been inclined to regard as oxides of organic radicals. Ether was in these countries regarded as an organic oxide, and this difference of view excited a ten years' strife, as an immediate result of which we may regard the discovery of a great number of com pounds which enriched science with innumerable important obser vations. No region of organic chemistry has been so thoroughly and so completely studied as the compounds connected with ether; and now, when the existence of organic oxides is no longer denied, the support of the opposite opinion has come to an end, although it can not be said that the question itself has been experimentally decided. If we compare in the light of our present knowledge the ammonia compounds with the ether compounds, we at once see that the opposing views were fundamentally the same in the two cases. The disputes took place because we were not at one as to the interpreta tion of the phenomena. The ether and ammonia compounds assume in fact the same form if amidogen is regarded as the unchange able radical of the ammonia compounds, and acetyl [Regnault's aldehydene] as the starting point of the ether compounds. The two sets of compounds differ only in so far as we must ascribe to acetyl the power of forming acids, a power which amidogen does not possess."
He then gives a table containing in two columns the ammonia and the ether compounds, in which C.,H 3 corresponds to NH 2 , Cyi., to NH 3 , and C 2 H 5 to NH 4 , and adds, "These formulæ require no explanation; they have been developed in order to show the extraordinary resemblance of the ammonia and the ether compounds, and to show why it was that many chemists regarded olefiant gas as the first member of the series of ether compounds. Both of the formerly antagonistic theories have, as may be easily seen, from this point of view the same foundation, and all further questions as to the truth of the one or other view is thus of course set at rest."
It was during the course of the controversy which then closed that Liebig had discovered aldehyde, chloral, and, simultaneously with Soubeiran, chloroform, besides numerous other substances of less general interest, and developed the theory now generally received of the formation of ether by the action of sulphuric acid on alcohol.
In the very short sketch given above of the discussion as to the constitution of ether, we mentioned that Liebig's ethyl theory was to some extent borrowed from Berzelius. Berzelius had suggested that ether was the oxide of a radical (Liebig's ethyl), but he was at first inclined to regard alcohol, not as the hydrate oxide of the same radical, but rather as the oxide of another one, which with our symbols would be written C 2 H fi . But there was a deeper difference than this between the radical theories of Berzelius and Liebig. This essential difference first clearly showed itself in the notes which Liebig added to two letters from Berzelius to Wöhler, published in the Annalen in 1839. In these letters Berzelius gives his views of the constitution of oxychlorides, with which he classes such bodies as trichloracetic acid. All those bodies he represents as compounds of oxides and chlorides, in harmony with the dualistic system.
Thus, instead of S0 2 C1 2 , C 2 HC1 3 2 , &c., he writes SCI 6 + 2S0 3 , C. 2 C1 6 +C S 3 , &c. (taking the anhydrous acid). In his second letter on Malaguti's chlorinated ethers he naturally arrives at formula of extreme complexity. In his notes Liebig states that he does not agree with Berzelius, and that the analogy first pointed out by Berzelius between inorganic and organic compounds and his theory of organic radicals had been a guiding star in a labyrinth in which previously no one could find the way. "But, while there are points of resemblance, there are very many points of difference; we should follow a theory as long as it gives us light and explains facts; up to a certain point the principles of inorganic chemistry help us in organic chemistry, beyond it they leave us, and produce instead of removing complications; beyond this point we require new principles."
These new principles were supplied by Liebig's radical theory. As Liebig showed, abstract discussions as to the truth of a theory are out of place in an experimental science; the question is not as to their essential truth but as to their practical fruitfulness. Do they help us to understand old and to discover new facts? If they do, the morrow may take thought for its own theories. These are not Liebig's words, but they seem to express his ideas.
Liebig early expressed his approval of Davy's views as to the constitution of acids, but he rarely used the language or the notation corresponding to that view. This divergence between his theory and his habitual language is interesting as showing that he held that the same truth may be expressed in more than one way, and that where no immediate point is to be gained it is well to employ the language best understood by those whom we address. In this, as in his preference for what was called the equivalent system of notation over that of Berzelius, he showed his sound practical judgment and common sense. We now see that the notation of Berzelius was nearer the truth, but its advantages could not be felt until chemistry had advanced further, and its retention would have led to complications of formulæ and obscuring of relations. The resemblance indeed of our present notation to that of Berzelius is to a great extent accidental, and the advance was hastened rather than hindered by what now looks like a retrograde step.
There is one other point which we have to mention under the present head. Liebig at once saw the importance of Graham's researches on the phosphates. He applied Graham's idea of polybasicity to organic acids, and satisfactorily proved, notwithstanding the opposition of Berzelius, that tartaric acid is dibasic and citric acid tribasic.
We have hitherto said nothing as to the relation of Liebig's theories to those at present held by chemists. On this subject a word may suffice. The great revolt against the radical theory led by Laurent and Gerhardt produced a long and acrimonious controversy. In that controversy Liebig took his part, and many hard and some unfair things were said by him. The controversy itself was of course the means of producing a vast amount of thorough research, and was thus, like all such contests, of direct use; it also led to the revision of all theoretical opinions from a totally new point of view. From this ordeal the radical theory has emerged, not very different in appearance. But it has undergone a profound change. Its foundations have been immensely strengthened, it has been to a great extent explained. Some chemists seem to think that this makes it an entirely new theory. We cannot share this view. Our reasons for believing in ethyl and benzoyl differ from the reasons adduced by Wöhler and Liebig only in this that we have arguments which they had not; their arguments remain. We now know something of the reason why such radicals exist; we can, to a certain extent, deduce their properties from those of the elements which they contain, but explanation is something different from refutation; the theory has grown, but still remains the same theory.
Liebig all his life showed a special predilection for the study of the immediate products of animal life. He investigated with untiring zeal the substances contained in urine and in the juice of flesh. In these researches he discovered many new substances, and cleared up doubts and difficulties as to their relations to one another and to other bodies. Late in life he expressed the most lively interest in Volhardt's synthesis of sarcosin and creatine, substances with the preparation of which he had long before been engaged. In this connexion it is right that we should mention his elaborate investigation on uric acid and its derivatives. This line of study led him to interest himself in the chemistry of food, and the importance of his work in this direction can scarcely be overestimated. We do not refer alone to his methods of preparing the extract of meat and the food for infants, which have perhaps spread his fame more widely than his strictly scientific work could have done; we refer rather to the influence which his analyses and calculations have had on medical opinion and practice. And this leads us to our fifth head, Liebig's influence on the application of chemistry to physiology, agriculture, and the arts.
Before Liebig undertook his investigations into the chemistry of vegetation, the views (they can scarcely be called theories) held as to the manner in which plants are nourished were vague in the extreme. The only point satisfactorily made out was that under the influence of light the green parts of plants are capable of decomposing carbonic acid, giving off oxygen and retaining the carbon. Saussure, to whose careful experiments the establishment of this fact, first noticed by Priestley, is mainly due, believed that the nitrogen of the plant came from soluble organic substances absorbed by the roots, and expressly says that the main use of ammonia in manure is as a solvent of humus, which he supposed to be one source of the carbon in plants; and, although the ashes of the various plants had been analysed, the importance of the mineral constituents of vegetables was not at all recognized. Liebig undertook the investigation of this question in 1840. He showed that the plant derives its nourishment partly from the air, partly from the soil; the carbonic acid and water, the ammonia and nitric acid, which he showed to be the sources of the plant's nitrogen, come from the atmosphere; while the potash, soda, lime, iron, magnesia, sulphuric acid, phosphoric acid, and silica come from the soil. No exhaustion can take place of the former, but the soil contains only a limited amount of the latter in a soluble state, and when this is used up the soil becomes barren. Not only so, but the absence of any one of the necessary substances makes the soil barren. He showed how manure acts by restoring these deficient ingredients, and how, when the land is left fallow, atmospheric influences decompose the insoluble minerals and supply the soil with what has been removed. He further showed that plants use and therefore remove from the soil the articles of plant food in various proportions,
