Wells $ Foote — One Hundred Years of Chemistry. 271 



heats of elements multiplied by their atomic weights give 

 a constant called the atomic heat. For instance, the 

 specific heats of sulphur, iron, and gold have been given 

 as 0-2026, 0-110, and 0-0324, while their atomic weights 

 are about 32, 56, and 197, respectively ; hence the atomic 

 heats obtained by multiplication are 6-483, 6-116, and 

 6-383. 



Further investigations showed that the atomic heats 

 display a considerable variation. Those of carbon, 

 boron, beryllium, and silicon are very low at ordinary 

 temperatures, although they increase and approach the 

 usual values at higher temperatures. More recent work 

 has shown, however, that the specific heats of other ele- 

 ments vary greatly with the temperature, almost disap- 

 pearing at the temperature of liquid hydrogen, and hence 

 possibly disappearing entirely at the absolute zero, where 

 the electrical resistance of the metals appears to vanish 

 likewise. , 



It has been found that most of the solid elements near 

 ordinary temperatures give atomic heats that are 

 approximately 64. Berzelius applied the law in fixing 

 a number of atomic weights, and its importance for this 

 purpose is still recognized. 



It may be mentioned here that two well-known Yale 

 men, W. G. Mixter and E. S. Dana, while students in 

 Bunsen's laboratory at Heidelberg in 1873, made deter- 

 minations of the specific heats of boron, silicon, and zir- 

 conium. This was the first determination of this con- 

 stant for zirconium, and it was consequently important 

 in establishing the atomic weight of that element. 



Isomorphism and Polymorphism. — Mitscherlich ob- 

 served in 1818 that certain phosphates and arsenates 

 have the same crystalline form, and afterwards he 

 reached the conclusion that identity in form indicates 

 similarity in composition in connection with the number 

 of atoms and their arrangement. This law of isomorph- 

 ism was of much assistance in the establishment of cor- 

 rect lormulas and consequently of atomic weights. For 

 instance, since the carbonates of barium, strontium, and 

 lead crystallize in the same form, the oxides of these 

 metals must have analogous formulas. From such con- 

 siderations Berzelius was able to make several improve- 

 ments in his atomic weight table of 1826. 



Mitscherlich was the first to observe two forms of 



