358 



On Double Salts of Nitrate of Silver. 



[June 21, 



sodium nitrate, which is the first to separate out, notwithstanding its 

 being by far the more soluble salt of the two. The solution obtained by 

 treatment of the two salts with insufficient water does not vary in com- 

 position, as is the case with the potassium salt, and this is known to be 

 a strong confirmation of the statement of the non-existence of a double 

 salt of silver and sodium. 



"With ammonium nitrate a double salt is formed corresponding to the 

 potassium salt. In this case, owing to the similar degree of solubility of 

 both constituents, it forms on the evaporation of a solution containing 

 the salts in the proportion of single equivalents. On treatment of the 

 salts with insufficient water for solution the same thing happens as with 

 the potassium salts. 



With lithium nitrate no double salt could be formed ; but on the evapo- 

 ration of a solution having the two salts in equivalent proportions, it is 

 the very soluble silver nitrate which is the first to separate out. Again, 

 the composition of the solution obtained by treating different amounts of 

 the dry salts with insufficient water to dissolve them gives in all cases a 

 liquid of constant composition. 



With lead nitrate no double salt was formed. On evaporation, in this 

 case, it is the least soluble, the lead nitrate, which is the first to separate 

 out. 



When two salts are together in solution, in some cases the more 

 soluble, in others the less soluble, will be the first to separate out on 

 evaporation ; this action appears to depend on the affinity of the salt for 

 water — its hygroscopic character, not its solubility. 



The distinctive character of the double salt is confirmed by its crystal- 

 line form, which is similar in the two cases of the potassium- and the 

 ammonium-silver salt. 



The system to which these crystals belong is the oblique ; the elements 

 of the crystal being 



a : b : c== 1*405 : 1 : 1-646 

 17=82° 22' 



1 1 . 1 = 37° 14', 101. 001= 45° 8', 111 . 1 = 45° 7'. 

 Other important angles are : — 



10 0*101 = 136° 20', 100 . 1 1 = 54° 19', 

 110.110= 71° 20', 010 . 011 = 31° 29J'. 



The faces of the forms 211 and 122 appear to be hemimorphously 

 developed. 



Averages occur parallel to the faces of the forms {101} {110} {101}. 



Optical character negative ; the optic axes lie in a plane perpendicular 

 to the plane of symmetry, their divergence for red light being about 

 4° 25', for blue light 13° 11'. The (horizontal) dispersion of the first 

 mean lines in the plane of symmetry throws the mean line for red rays 



