§ =92.= Uranium and thorium differ in one important respect from radium, inasmuch as the first product of the decomposition of the uranium and thorium atoms is in both cases solid. Sir William Crookes[108] was able to separate from uranium salts by chemical means a small quantity of an intensely radioactive substance, which he called Uranium X, the residual uranium having lost most of its activity; and M. Becquerel, on repeating the experiment, found that the activity of the residual uranium was slowly regained, whilst that of the uranium X decayed. This is most simply explained by the theory that uranium first changes into uranium X. It has been suggested that radium may be the final product of the breaking up of the uranium-atom; at any rate, it is quite certain that radium must be evolved in some way, as otherwise there would be none in existence--it would all have decomposed. This suggestion has been experimentally confirmed, the growth of radium in large quantities of a solution of purified uranyl nitrate having been observed. Uranium gives no emanation. Thorium probably gives at least three solid products--Meso-thorium, Radio-thorium, and Thorium X, the last of which yields an emanation resembling that obtained from radium, but not identical with it. [108] Sir WILLIAM CROOKES, F.R.S.: “Radio-activity of Uranium,” _Proceedings of the Royal Society of London_, vol. lxvi. (1900), pp. 409 _et seq._ The Radium Emanation. § =93.= We must now more fully consider the radium emanation--a substance with more astounding properties than even the radium compounds themselves. By distilling off the emanation from some radium bromide, and measuring the quantities of heat given off by the emanation and the radium salt respectively, Professors Rutherford and Barnes[109] proved that nearly three-fourths of the total amount of heat given out by a radium salt comes from the minute quantity of emanation that it contains. The amount of energy liberated as heat during the decay of the emanation is enormous; one cubic centimetre liberates about four million times as much heat as is obtained by the combustion of an equal volume of hydrogen. Undoubtedly this must indicate some profound change, and one may well ask, What is the ultimate product of the decomposition of the emanation? [109] E. RUTHERFORD, F.R.S., and H. T. BARNES, D.Sc.: “Heating Effect of the Radium Emanation,” _Philosophical Magazine_ [6], vol. vii. (1904), pp. 202 _et seq._ The Production of Helium from Radium.