338 Materials for a History of ike Prussiates. 



of a maximum and minimum; and if the mixture of yellow 

 and blue sometimes presented to us by these residues is not 

 green, as might be expected, it is because the yellow oxide 

 always covers these remains of blue in a very great excess j 

 at least I never found the latter to exceed one or two hun- 

 dredth parts. 



I now proceed to the second case. A residue, if well 

 pulverized, may contain no vestiges of blue, but it easily 

 retains the two salts mentioned above. If we apply an acid 

 to it in this state, both give blue in abundance. We shall 

 by and by examine the particular mixture of these two salts ; 

 but if it has been carefully washed, the acids can no longer 

 produce any salts from it. Indeed, nothing is more tedious 

 than this washing, for I was obliged to renew the boiling 

 water at least 20 times successively upon a single drachm of 

 residue before I obtained it completely pure : but when we 

 at last succeed, the acids dissolve it without giving blue: 



When these residues effervesce with the acids, it is be- 

 cause they contain carbonates of potash or of lime. By 

 washing, the first is carried off; by the application of vinegar 

 after washing, we discover the second. Thus it is not the 

 red oxide which occasions this effervescence: it is not sus- 

 ceptible of being combined with the carbonic acid ; it cannot 

 therefore take it from potash in exchange for the prussic 

 acid which it receives from it. In nature, as well as in art, 

 the oxide of iron at the minimum solely may be united with 

 carbonic acid. \ 



One pound of blue of commerce of a fine quality gave 

 pearly nine ounces and a half of crystallized prussiate of pot- 

 ash. It is by no means rare to find in the mother waters, 

 when left to themselves, truncated octahedrons one inch in 

 diameter. When this blue carries sulphuric acid with it, 

 there must be at least four crystallizations, in order to puro-e 

 the prussiate of the whole sulphate of potash. These mother 

 waters contain alumine sometimes in abundance, sulphate 

 and phosphate of potash, ferruginous alkaline carbonate. By 

 this we may judge why, in analyses, it is of importance to 

 use crystallized prussiate, and not simple lixiviums of Prus- 

 sian blue, as formerly made. The prussiate of potash is 



unalterable 



