RADIOACTIVE LEAD RICHARDS. 207 



not made manifest as heat. These facts have been proved beyond 

 doubt by the brilliant work of Professors Becqnerel, Marie Curie, P. 

 Curie, Sir Ernest Rutherford, and others. 



The nature of the rays, and of the highly interesting evanescent 

 transition products and their relation to one another, is too complex for 

 discussion now. We are concerned rather with the nature of the more 

 permanent of the substances concerned — especially with the starting 

 point, uranium (possessing the heaviest of all atoms) , radium, and 

 the lead which seems to result from their disintegration. Omitting 

 the less stable transition products, the most essential outcomes are 

 roughly indicated by a sort of genealogical tree herewith shown : 



HYPOTHESIS CONCERNING THE DISINTEGRATION OF URANIUM. 



Uranium 



I \ 3 Helium 

 Radium 



| \ 1 Helium > 

 Emanation 



i \ 4 Helium J 

 Lead (Isotopic) 



Thus each atom of uranium is supposed to be converted into radium 

 by losing three atoms of helium, and each atom of radium is sup- 

 posed to be converted into a kind of lead by losing five more, as 

 already stated. 



If uranium can thus disintegrate, should we call it an element, 

 and should we call its smallest particles atoms? The answers de- 

 pend upon our definition of these two words. If the word " ele- 

 ment " is supposed to designate a substance incapable of disintegra- 

 tion, apparently it should not be applied to uranium ; neither should 

 the word " atom " be applied to the smallest conceivable particles of 

 this substance. But no one would now maintain that any element 

 is really incapable of disintegration. A method of still retaining 

 the terms in this and analogous cases is to define an element as " a 

 substance which has not yet been decomposed artificially," that is 

 to say, by the hand of man — and an atom as " the smallest particle 

 of such a substance, inferred from physicochemical behavior." The 

 atom, then, is not to be considered as wholly indivisible, but only as 

 indivisible (or at least as not yet divided) by artificial means. For, 

 as in the case of radium, the disintegration of uranium can not be 

 hastened or retarded by any known earthly agency. So long as it 

 stays intact, the atom of uranium 'behaves quantitatively in the same 

 fashion as any other atom: Dalton's laws of definite and multiple 

 combining proportions apply without exception to its compounds. 

 In this connection one should remember that the atomic theory, as a 

 whole, including Dalton's and Avogadro's generalizations, is not 



