PERIODIC ARRANGEMENT OF ELEMENTS 175 



pounds are termed, would, if they could exist, decompose 

 with evolution of heat. Such a decomposition appears to 

 present analogy with the change which an element like 

 radium is undergoing. It is in process of change into 

 other elements of lower atomic weight ; and in changing, 

 it evolves heat, in amount enormously greater than that 

 produced by any change of a compound into a mixture of 

 simpler compounds. But the matter is complicated by 

 another phenomenon that of discharging with almost 

 inconceivable velocity particles which appear, according 

 to J. J. Thomson, to be identical with negative electricity. 

 These ' corpuscles/ as they have been termed, embed them- 

 selves in the vessel in which the radio-active body is 

 confined ; and, owing to their extreme minuteness, they 

 may even pass through the walls of the containing vessel. 

 Indeed the opposition of their passage has been shown to 

 depend merely on the density of the matter of which the 

 confining walls are composed ; gold, which is denser than 

 lead, stops their passage better than lead; for a similar 

 reason lead is better than iron, iron better than glass, and 

 so on. Thomson has calculated that the mass of one such 

 particle is approximately one-thousandth of that of an 

 atom of hydrogen. 



This new chemistry is just at its commencement. It 

 dates from 1896, when Becquerel showed that compounds 

 of uranium evolved some sort of radiation which would 

 impress a photographic plate. It is still too early to 

 formulate any definite statement relating to its connection 

 with the irregularity in the numerical sequence of the 

 atomic weights ; yet it may be permissible to speculate, 

 aided by the recent discoveries. When two elements 

 combine, heat is generally evolved ; now heat is only one 

 form of energy, and the combination of elements may be 

 so carried out as to be accompanied by other kinds of 

 energy for instance, by the production of an electric. 



