272 Wells & Foote — One Hundred Years of Chemistry. 



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. 



In some cases isomorphism takes place to a greater or 

 less extent between substances which are not chemically 

 similar, and this brings about a variation in composition 

 which at times has caused confusion. For instance, the 

 mineral pyrrhotite has a composition which usually 

 varies between Fe 7 S 8 and FenS 12 , and both these formu- 

 las have been assigned to it. It was recently shown by 

 Allen, Crenshaw and Johnston in this Journal (33, 169, 

 1912) that this is a case where the compound FeS is 

 capable of taking up various amounts of sulphur 

 isomorphously. 



The idea of solid solution was advanced by van't Hoff 

 to explain the crystallization of mixtures, including cases 

 of evident isomorphism. This view has been widely 

 accepted, and it has been particularly useful in cases 

 where isomorphism is not evident. Solid solution 

 between metals has been found to be exceedingly com- 

 mon, many alloys being of this character. A case of 

 this kind was observed by Cooke and described in the 

 Journal (20, 222, 1855). He prepared two well-crystal- 

 lized compounds of zinc and antimony to which he gave 

 the formulas Zn 3 Sb and Zn 2 Sb, but he observed that 

 excellent crystals of each could be obtained which varied 

 largely in composition from these formulas. As the two 

 compounds were dissimilar in their formulas and crys- 

 talline forms, Cooke assumed that isomorphism was 

 impossible and concluded "that it is due to an actual 

 perturbation of the law of definite proportions, produced 

 by the influence of mass." We should now regard this 

 as a case of solid solution. 



