ONE HUNDRED YEARS OF CHEMISTRY 301 



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 formulas 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 

 sulphur crystals, and from this and other cases of 

 dimorphism or of polymorphism it became evident that 

 analogous compounds were not necessarily always iso- 

 morphous, a circumstance which has restricted the 

 application of the law to some extent. 



Besides its application in fixing analogous formulas, 

 the law of isomorphism has come to be of much practical 

 use in the understanding and simplification of the formu- 

 las for minerals, for these natural crystals very often 

 contain several isomorphous compounds in varying pro- 

 portions, and an understanding of this "isomorphous 

 replacement," as it is called, makes it possible to deduce 

 simple general formulas for them. 



19 



