302 A CENTURY OF SCIENCE 



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 Fe n S 12 , and both these formu- 

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

 Allen, Crenshaw and Johnston in the 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 Hofd 

 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. 



A Lack of Confidence in Avogadro's Principle. One 

 reason why chemists were so slow in arriving at the 

 correct atomic weights and formulas was a partial loss 

 of confidence in Avogadro's principle. About 1826 the 

 young French chemist Dumas devised an excellent 

 method for the determination of vapor densities at high 

 temperatures, and his results and those of others showed 

 some discrepancies in the expected densities. For 

 example, the vapor density of sulphur was found to be 

 about three times too great, that of phosphorus twice too 

 great, that of mercury vapor and that of ammonium 



