xvi ATOMIC WEIGHTS OF THE METALS 371 



crystals may still be formed, the angles of which are a 

 compromise between those of the constituents. 



Applications of the law of isomorphism. In the 

 isomorphous phosphates and arsenates, 75 grams of arsenic 

 replace 31 grams of phosphorus without altering the crystal- 

 line form. Since 31 is the atomic weight of phosphorus, 

 the Law of Isomorphism indicates that 75 is the atomic 

 weight of arsenic, thus confirming the value arrived at by 

 other methods. A similar resemblance appears in the 

 crystalline properties of arsenic, antimony and bismuth. 

 In the isomorphous compounds of these elements, 75 

 grams of arsenic are replaced by 120 grams of antimony, 

 and 120 grams of antimony by 208 grams of bismuth, 

 without producing any large alteration of crystalline form. 

 In this way the Law of Isomorphism leads up from P = 3i, 

 through As = 75 to Sb = 1 20 and Bi = 208. 



In 1828 Mitscherlich showed (Pogg. Ann., 1828, 12, 138) 

 that the selenates and sulphates of sodium and silver formed 

 an isomorphous series in which 23 parts of sodium were 

 replaced by 108 parts of silver, whilst 32 parts of sulphur 

 were replaced by 79 parts of selenium. This isomorphism 

 served to establish the atomic weight of selenium. 



Isomorphism of manganese and chromium with 

 chlorine and sulphur. Two interesting cases of isomor- 

 phism were described in detail by Mitscherlich in 1832 

 (Pogg. Ann., 1832, 25, 287-302). He showed that the red 

 potassium permanganate was isomorphous with potassium 

 perchlorate. In these compounds 55 parts of manganese 

 replace 35-5 parts of chlorine; 55 is therefore the atomic 

 weight of manganese. On the other hand, the green 

 potassium manganate was isomorphous with the chromate, 

 selenate and sulphate; the equivalent quantities, S 32, Se 79, 

 Cr 52, Mn 55, are therefore the atomic weights of these 

 elements. 



Mitscherlich had already proved the isomorphism of the 



B B 2 



