Wells & Foote — One Hundred Years of Chemistry. 287 



have led to modified views in regard to the stability of 

 the elements in general. There has been little or no 

 proof obtained that any artificial transmutation of the 

 elements is possible, but the spontaneous transformation 

 of the radioactive elements brings forward the possibility 

 that other elements are changing imperceptibly, and that 

 a state of evolution exists among them. All of the radio- 

 active changes that we know proceed from higher to 

 lower atomic weights, and we are entirely ignorant of the 

 process by which uranium and thorium must have been 

 produced originally. 



Since radioactive changes have been found to be 

 accompanied by the release of vast amounts of energy, 

 compared with which the energy of chemical reactions is 

 trivial, a new aspect in regard to the structure of atoms 

 has arisen, — they must be complex in structure, the seats 

 of enormous energy. 



The determination of the amount of radium in the 

 earth's crust has indicated that the heat produced by it is 

 amply sufficient to supply the loss of heat due to radia- 

 tion, and this source of heat is regarded by many as the 

 cause of volcanic action. The sun's radiant heat also 

 has been supposed to be supplied by radioactive action, 

 so that the older views regarding the limitation of the 

 age of the earth and the solar system on account of loss of 

 heat have been considerably modified by our knowledge 

 of radioactivity. 



Physical Chemistry. 



The application of physical methods as aids to chem- 

 ical science began in early times, and some of these, such 

 as the determinations of gas and vapor densities, specific 

 heats, and crystalline forms have been mentioned already 

 in this article. Within recent times physical chemistry 

 has greatly developed and a few of its important achieve- 

 ments will now be described. 



Molecular Weight Determinations. — Gas and vapor 

 densities in connection with Avogadro's principle, 

 formed the only basis for molecular weight determina- 

 tions until comparatively recent times. The early 

 methods of Gay-Lussac and Dumas for vapor density 

 were supplemented in 1868 by the method of Hofmann, 

 whereby vapors were measured under diminished pres- 



