96 HISTORY OF CHEMISTRY. [LECTURE VI. 



with his, but these do not merit any confidence, as later 

 observers (Regnault 25 and Kopp 26 ) have found other numbers 

 which agree better. 



I may take this opportunity to state that Neumann showed, 

 in 1 83 1, 27 that Dulong and Petit's law may also be extended 

 to compounds of analogous composition ; that is to say, the 

 specific heats of these compounds multiplied by their equiva- 

 lent weights (as Neumann calls them) give equal products. 

 The law was proved, in particular, for the carbonates and the 

 sulphates. 



Before I pass on to Mitscherlich's interesting results, I wish 

 to make some historical remarks by way of preface. With 

 Hauy, the crystalline form (primative form) was an important 

 characteristic for the determination of the nature of a substance; 

 difference of form being, in his view, a ground for assuming a 

 different composition, 28 although Berthollet contested this. 29 

 Gay-Lussac observed, in 1816, that crystals of potash alum 

 increase in volume in a solution of ammonia alum, without 

 altering in shape. 30 Beudant 31 also made very interesting 

 statements in this connection; and, as early as 1817, J. N. v. 

 Fuchs 32 drew attention to the similarity of the crystalline forms 

 of arragonite, strontiamte, and cerussite. Gehlen stated that 

 he had succeeded in preparing crystals of alum with soda. 33 



These were isolated observations, which were insufficient 

 to overthrow Hauy's doctrine, and which only attained to any 

 importance through Mitscherlich's discovery of isomorphism. 34 

 In 1820, Mitscherlich established the fact that the corre- 

 sponding phosphates and arseniates, with the same number of 

 atoms of water, possess the same crystalline form, so that even 

 the secondary forms coincide. Even at that time, the same 

 number of atoms was assumed to be present in both acids, and 



25 Ann. Chim. [2] 73, 5; ibid. [3] i, 129; 9, 322 etc. - 6 Annalen. 

 Supplementary Vol. 3, 291. 27 Pogg. Ann. 23, I. w Traite de mineralogie. 

 29 Stat. Chim. I, 433 ; E. I, 432. 30 Kopp, Geschichte. 2, 406. 31 Ann. 

 Chim. [2] 4, 72 ; 7, 399 : 8, 5 ; 14, 326. 32 Schweigger's Journal. 19, 133. 

 33 Ibid. 15, 383, Note. M Ann. Chim. [2] 14, 172; 19, 350; 24, 264 

 and 355. 



