368 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



Kopp's rule gives expression to the fact that lower values 

 are found for the molecular heats of compounds containing 

 elements such as boron, carbon and sulphur, of which the 

 atomic heats are abnormally low. 



C. THE LAW OF ISOMORPHISM. 



Mitscherlich (1819) discovers isomorphism. In select- 

 ing atomic weights, Berzelius adopted the rule that com- 

 pounds which resemble each other closely should be repre- 

 sented by similar formulae. A remarkable illustration of 

 this close resemblance between compounds of similar com- 

 position was discovered in 1819 by Mitscherlich, a pupil 

 of Berzelius. " He examined the acid phosphate and arsenate 

 of potassium, which are now represented by the formulae 

 KH 2 PO 4 and KH 2 AsO 4 , and found that 



" these salts are composed of the same number of atoms . . . , 

 and only differ from one another in that the radical in one 

 is phosphorus and in the other arsenic. The crystalline 

 form of these two salts is the same . . . ; for not only the 

 primitive form, but all the varieties, resemble each other so 

 closely in the size, number, and angles of the faces, that it 

 is quite impossible to find any difference, even in the 

 characters which appear to be quite accidental " (" On the 

 Relation between Crystalline Form and Chemical Propor- 

 tions," Ann. Chim. Phys., 1820, 14, 172 and 173). 

 A similar analogy of form was detected in 



the sodium salts { ^J^O } and 



the ammonium salts j /^S^S 2 An I 

 I ViNrlJrlgAS^ J 



In each of these cases 75 parts of arsenic could take the 

 place of 31 parts of phosphorus, without producing any alter- 

 ation of crystalline form. 



In the second part of the memoir (Ann. Chim. Phys., 

 1821, 19, 350) Mitscherlich described as ISOMORPHOUS 



