326 A CENTURY OF SCIENCE 



the reacting substances, which may be uncertain or un- 

 known, and chiefly on this account it has, like the phase- 

 rule, often only a qualitative significance. 



The phase rule has served as a most valuable means 

 of classifying systems in equilibrium and as a guide in 

 determining the possible conditions under which such 

 systems can exist. As illustrations of its practical appli- 

 cation, van't Hoff used it as an underlying principle in 

 his investigations on the conditions under which salt 

 deposits have been formed in nature, and Rooseboom was 

 able by its means to explain the very complicated rela- 

 tions existing in the alloys of iron and carbon which form 

 the various grades of wrought iron, steel and cast iron. 



Thermochemistry. This branch of chemistry has to 

 do with heat evolved or absorbed in chemical reactions, 

 It is important chiefly because in many cases it furnishes 

 the only measure we have of the energy changes involved 

 in reactions. To a great extent, it dates from the dis- 

 covery by Hess in 1840 of a fundamental law which states 

 that the heat evolved in a reaction is the same whether it 

 takes place in one or in several stages. This law has 

 made it possible to calculate the heat values of a large 

 number of reactions which cannot be determined by 

 direct experiment. 



Thermochemistry has been developed by a compara- 

 tively few men who have contributed a surprisingly 

 large number of results. Favre and Silbermann, begin- 

 ning shortly after 1850, improved the apparatus for cal- 

 orimetric determinations, which is called the calorimeter, 

 and published many results. At about the same time 

 Julius Thomsen, and in 1873 Berthelot, began their 

 remarkable series of publications which continued until 

 recently. Thomsen 's investigations were published in 

 1882 in 4 volumes. It is probably safe to say that the 

 greater part of the data of thermochemistry was obtained 

 by these two investigators. The bomb calorimeter, an 

 apparatus for determining heat values by direct combus- 

 tion, was developed by Berthelot. The recent work of 

 Mixter at Yale, published in the Journal, and of Rich- 

 ards at Harvard should be mentioned particularly. 

 Mixter 's work in this field began in 1901 (12, 347). 

 Using an improved bomb calorimeter, he has developed a 



