338 Mr. M. Carey Lea's Notes 



instructive. When 100 cubic centim. of solution are poured 

 into 100 cubic, centim. of water to which 3 cubic centim. of 

 sulphuric acid have been previously added, a dark red precipi- 

 tate falls, which, when dry, especially in films, is blue. The 

 mixed liquid from which the precipitate is formed is acid. 

 Increasing the proportion of acid to 4, 5, and 6 cubic centim. 

 successively, the substance obtained has a green surface-colour, 

 becoming more yellowish green in proportion as the acid is 

 increased in quantity. With 1\ cubic centim. the substance 

 no longer dries green, but yellow. Increased proportions of 

 acid produce substances drying with a coppery shade. 



It will be seen that from a single solution, and using one 

 substance only as a precipitant, we can obtain the whole range 

 of different forms of allotropic silver by simply varying the 

 proportions of the precipitant. 



That these forms of silver should subsist in the presence of 

 sulphuric acid in excess is remarkable. For the most part 

 the presence of this acid tends to quickly convert allotropic 

 to ordinary silver. For example, bright yellow allotropic 

 silver obtained with ferrous tartrate was washed on a filter 

 with water containing -^^ its volume of sulphuric acid : in 

 two or three hours the entire mass was converted into grey 

 ordinary silver. 



It is observable that the substances precipitated with the 

 least acid have a very splendid lustre, and that this lustre 

 diminishes steadily as the proportion of acid is increased. 

 Up to 6 to 100 cubic centim. the effect is hardly noticeable, 

 after that it becomes more marked. 



But we can also obtain the converse of this reaction. Just 

 as the solution which naturally would yield the blue product 

 can be made to yield the yellow b}^ the presence of excess of 

 strong acid, so the solution which normally yields the yellow 

 substance may be made to produce blue (or rather green) 

 silver by adding alkali. Thus a mixture of dilute solutions of 

 ferrous sulphate and of Rochelle salt added to mixed solutions 

 of silver nitrate and of Rochelle salt results in the formation 

 of gold-coloured silver. But if we add a little sodium hy- 

 droxide, either to the iron solution or the silver mixture, we 

 shall get a bluish-green product, whose properties show that 

 it belongs to the blue class and not to the yellow. Even if a 

 solution of the hydroxide is added immediately after the iron 

 solution has been poured into the silver, the result is the same. 



There is, therefore, a well-marked tendency of acids to give 

 rise to the formation of the yellow product and of alkalies to 

 the blue. But this is a tendency only. Both substances can 

 be produced from neutral solutions, and slight changes are 



