204 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



active deposits and helium. The principles of thermodynamics came 

 safe and sound out of the fray, but that of the immutability of the 

 elements was moribund, 



"We distinguish between elements which show radioactive charac- 

 teristics and those which do not. It is nevertheless true that the 

 radioactive ones are elements, and therefore we can not hold that 

 elements are immutable. 



The theory of the disintegration of radioactive bodies, due to Sir 

 Ernest Rutherford, justly dominates modern science. The ideas 

 Avhich he postulates are to-day solidly founded. The phenomenon 

 of spontaneous radioactivity is interpreted as the projection out of 

 the atomic nucleus of an electron, a helium ion. 



The electron, the carrier of the ^ radiation in which matter and 

 negative electricity are identical, is now the smallest mass of matter 

 known to science. Its mass at rest is one thousand eight hundred 

 times less than that of the hydrogen atom. It plays a part in a 

 great number of material phenomena not related to radioactivity. 

 There is no chemical element which can not liberate electrons detect- 

 able by electrometric means. It is apparently a universal constitu- 

 ent of matter. 



The helium ions which constitute the a rays have a mass approxi- 

 mately that of a helium atom and each carries two elementary elec- 

 tric charges equal except in sign to that of the electron. They can 

 not be considered a universal constituent of matter, since all the 

 atomic weights are not even approximately multii)les of four. There 

 is a tendency to consider them as being themselves composed of four 

 hydrogen nuclei (or protons, actual atoms of positive electricity), 

 representing four charges, and associated in the helium ion with 

 two electrons for the necessary intra-atomic binding field of force. 

 Hence the hydrogen ion or proton may be considered the second 

 universal constituent of matter. Electrons and protons suffice to 

 make up the assemblages which are our chemical atoms. This man- 

 ner of postulating the building of the universe is admirable. It 

 satisfies at the same time both the scientist and the philosopher. 

 Its scientific character is undeniable. The existence of electrons is 

 certain; they have been isolated by the most diverse methods, to 

 which I believe there can be no serious objection. 



The reality of protons is less certain. They must go through sev- 

 eral small formalities before they are accepted in scientific society. 

 They are tolerated, since their existence fills certain needs. 



Only a little Avhile ago several physicists were disposed to postu- 

 late that matter was composed exclusively of electrons. But it is 

 difficult to conceive how masses of negative electricity could build 

 neutral atoms and molecules. Positive electricity remains an 

 enigma. It seems logically necessary that material negative par- 



