342 Materials for a History of the Pnissiates. 



mospheric air, the precipitate resulting from it assumes the 

 appearance of a piece of parsley cheese. 



Fill two glasses with boiling water : put a few drops of 

 prussiate into the one, and an equal quantity into the se- 

 cond : add to the latter some drops of hvdrosulphuret of 

 potash or of ammonia. These two glasses being thus pre- 

 pared, drop some nitrate of iron into them : the first yields, 

 as might be expected, a complete blue ; but the second pre- 

 sents the amusing spectacle of a precipitate which, al- 

 though blue at first, rapidly quits this colour to become 

 white. The theory of these facts is so evident that 1 shall 

 not dwell upon it, nor shall I repeat all the other experi-^ 

 ments mentioned in my first memoir, in order to establish 

 the existence of the two pnrssiates of iron. If the prussiate 

 at the minimum has no colour when it is not affected by the 

 atmosphere, we find that the green sulphate when dried has 

 none also. The absence of colour in one of these salts is 

 surelv not more astonishing than its absence in the other; 

 and finally, if we obtain red oxide by applying the alkalis 

 to the blue prussiates, it is on the contrary black oxide 

 which we extract from the white prussiate. But these dif- 

 ferences, which theory previously points out, perfectly co- 

 incides with those exhibited by the red and green sulphates 

 under similar circumstances. 



In my first memoir I advised the pouring of the prussiate 

 of potash upon the sulphate in a flask, in order to avoid, as „ 

 much as possible, the- mixture of the air, but I succeeded 

 verv imperfectly : in the first place because cold liquids al- 

 ways brins; some air along with, them; and secondly, be- 

 cause I had not thought of sulphuretted hydrogen for purging 

 them. I was not then acquainted with the way in which it 

 acted with these salts. 



If, for example, we dilute the solution of green sulphate 

 with three or four times its volume of sulphuric or muriatic 

 acid, the excess of these acids does not in the least change 

 the result. ■ The white prussiate only wanting colour on ac- 

 count of the want of oxygen, we should think that additions 

 like these are not made for the purpose of giving it. Acids 



more 



