318 A CENTUEY OF SCIENCE 



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- 

 sure over mercury. In 1878 Victor Meyer introduced a 

 simpler method depending upon the displacement of air 

 or other gas by the vapor in a heated tube. As refrac- 

 tory tubes, such as those of porcelain or even iridium, 

 could be used in this method, molecular weights at 

 extremely high temperatures were determined with inter- 

 esting results. For instance, it was found that iodine 

 vapor, which shows the molecule I 2 at lower tempera- 

 tures, gradually becomes monatomic with rise in tem- 

 perature, that sulphur vapor dissociates from S 8 to S 2 

 under similar conditions, and that most of the metals, 

 including silver, have monatomic vapors. 



In 1883 and later it was pointed out by Eaoult that the 

 molecular weights of substances could be found from the 

 freezing points of their solutions, but this method was 

 complicated from the fact that salts, strong acids and 

 strong bases behaved quite differently from other sub- 

 stances in this respect, and allowances had to be made for 

 the types of substances used. The complication was 



