144 ISOMORPHISM. 



silver. The term ammonium is applied to a hypothetic com- 

 pound of one atom of nitrogen and four of hydrogen (NH 4 ), 

 which is, therefore, certainly not an elementary body, and 

 probably not even a metal, but which is conveniently assimi- 

 lated in name to potassium, as these two bodies occupy the 

 same place in the two great classes of potash and ammonia 

 salts, between which there is the mostcomplete isomorphism. Po- 

 tassium and ammonium themselves are, therefore, isomorphous. 

 The sulphates of soda and silver are similiform, and hence also 

 the metals sodium and silver ; but their isomorphism with the 

 preceding pair is not so clearly established. Soda replaces po- 

 tash in soda-alum, but the form of the crystal is the common 

 regular octohedron ; nitrate of potash has also been observed 

 in microscopic crystals having the arragonitic form of nitrate of 

 soda,* which is better evidence of isomorphism, although not 

 beyond cavil as the crystals were not measured. There are 

 also grounds for believing that potash replaces soda in equiva- 

 lent quantities in the mineral chabasie, without change of form. 

 The probable conclusion is that potash and soda are isomor- 

 phous, but that this relation is concealed by dimorphism, ex- 

 cept in a very few of their salts. 



This class is connected in an interesting way with the 

 the other classes through the second. The sub- sulphuret of 

 copper and the sulphuret of silver appear to be isomorphous, 

 although two atoms of copper are combined in the one sul- 

 phuret and one atom of silver in the other, with one atom of 

 sulphur ; their formulae being 



Cu 2 S and Ag S. 



Are then two atoms of copper isomorphous with one atom of 

 silver ? In the present state of our knowledge of isomorphism, 

 it will be wise to admit that they are. 



The fourth class will thus stand apart from the second which 

 is represented by copper, and also from the other classes con- 

 nected with the second, in so far as one atom of the present 

 class is equivalent to two atoms of the other classes in the 

 production of the same crystalline form. This discrepancy may 

 be at once removed by halving the atomic weight of silver, 

 and thus making both sulphurets to contain two atoms of me- 



* Frankenheim in Poggendorff' s Annalen, vol. ^0, page 447. See also a paper 

 by Professor Johnston on the received equivalents of potash, soda and silver ; 

 Phil. Mag. third series, vol. 12, p. 324. 



