beyond which they were unable to go, no matter how much lead was used. Strutt and others then found that for electroscopes of the same dimensions, the amount of ionization depended on the material forming the walls. For vessels of the same shape and size, lead walls gave the greatest amount of ionization, tin and iron considerably less, aluminium and glass the least of all. Strutt found that different specimens of the same metal gave a different ionization and he therefore concluded that the radioactivity of the metals was probably due to a common impurity.
Patterson then tried using different gases and found that the ionization was proportional to the density. This fact is strong evidence that the ionization is not spontaneous within the gas, but is due to a radiation from the walls of the vessel. Patterson also found for the given vessel which he used (30 cm. in diameter and 20 cm. long) that the current through the gas was independent of the pressure above 300 mm. of mercury and varied directly as the pressure below 80 mm. The ionization was found independent of the temperature up to 450° C. That the ionization was related to the pressure as stated above would indicate that above 300 mm. of mercury all the radiation was absorbed, whereas below 80 mm. it was not all absorbed.
The most complete work on the radiations from the metals and their salts has been done by Campbell. In experiments on the radiations from the metals, Campbell used an aluminium-lined box. Inside this was a wire gauze cage containing a gauze electrode. The cage would allow the admission of radiations, but not of ions. Then by placing two sheets of metal so as to radiate into the cage, one sheet being arranged to slide back and forth, it was possible to measure the ionization produced at different distances of this sliding sheet from the cage. The curve which was plotted from the values of the ionization and the distances gave the values of various constants from which it was possible to determine the values given in a table which is shortly to follow. Before considering this table it is needful to say that the curves indicated (when the external penetrating radiation was cut off): (1) an easily absorbable radiation from the sheets of metal placed aside of the cage; (2) a more penetrating radiation from the same; and (3) the radiation from the gauze cage. When the external penetrating radiation was not screened off, the curves showed in addition an ionization due to (4) the external pentrating radiation; (5) to the penetrating radiation excited by it; and (6) to the easily absorbable radiation also produced. In the table, is Bragg's constant for the intrinsic absorbable radiations, a constant which corresponds to the range of the particles of the radioactive elements; is the number of ions produced per second by the intrinsic absorbable radiation from one square centimeter of the surface of the metal, when totally absorbed in air; is the coefficient of absorption of the easily absorbable secondary radiation; is the number of ions produced per cubic centimeter by
