LECTURE VI.] HISTORY OF CHEMISTRY. 97 



thus Mitscherlich arrived at the idea that it was the similarity 

 of atomic constitution which gave rise to the identity of form. 

 And he really succeeded in confirming this opinion by a series 

 of facts. He called those substances isomorphous, which 

 exhibit the same crystalline form in corresponding compounds, 

 and which, since they can crystallise together, replace one 

 another in indefinite proportions. He pointed out the isomor- 

 phism of selenic acid and sulphuric acid ; that of magnesia, 

 zinc oxide, nickelous oxide, ferrous oxide, etc., in their neutral 

 sulphates, etc. ; that of alumina, ferric oxide, and manganic 

 oxide. He also showed that Beudant's observation, in ac- 

 cordance with which iron vitriol and zinc vitriol (two salts of 

 different crystalline form, and containing different proportions 

 of water) crystallise together, depends upon the fact that the 

 proportion of water in the one compound is changed, and 

 becomes the same as that in the other. 



Other observers confirmed Mitscherlich's view by means of 



many observations, 35 so that, at that time, much stress was laid 



upon the crystalline form of substances ; and chemists supposed 



that they possessed, in this character, an excellent means of 



obtaining information regarding their atomic constitution. It 



was Berzelius, in particular, who, instantly recognising the 



bearing of the great discovery, applied it to the extension of 



his system. Isomorphism led him to the following rule : 36 



III. When one substance is isomorphous with another in 



which the number of atoms is known, then the 



number of atoms in both is known, because 



isomorphism is a mechanical consequence of 



similarity of atomic construction. 



Guided by these rules, Berzelius endeavours to determine 

 the number of atoms contained in a compound, and from this, 

 he can then deduce the atomic weights. He is quite conscious 



35 Literature in the article " Isomorphismus " in the Handworterbuch 

 der Chemie, edited by Liebig, Poggendorff, and Wohler. In the article 

 " Isomorphie" in the second edition of this Dictionary, Arzruni enters par- 

 ticularly into the more recent development of the doctrine of isomorphism. 

 36 Lehrbuch. 3, part I, 91. 



G 



