Materials far a History of the Prussiates. 337 



of potash may be attached to Prussian blue, so strongly as 

 to resist the washing to a certain degree. I do not think 

 with him, however, that the surplus of this salt should be 

 considered as an element essential to it, for the Prussian 

 blue which has been well prepared, leaves no trace of saline 

 matter in the residue of its distillation. 



Prussian blue, prepared without alum, is cupreous like 

 fine indigo. It loses 0*45 only by combustion. Its residue 

 is red oxide without any mixture of extraneous matters. 



Action of the ^Alkalis. — Prussian blue, when tried with 

 caustic potash, leaves a residue which is only red oxide con- 

 founded with alumine. The shade is that of kermes, if the 

 blue be of a good quality : on the contrary, it is pale and 

 earthy if it has been surcharged with alumine ; so that we 

 may judge of its nature extremely well from the colour of 

 the residue. 



The acids, when applied to a residue properly washed, 

 extract no colour iron it : this shows that we may in a 

 single operation rob the Prussian blue of all its acid; but 

 for this purpose it must be finely pulverized, which is ex- 

 tremely difficult to accomplish. If we throw some drops of 

 alkali into water coloured by blue freshly precipitated, the 

 colour is completely discharged : in this case the oxide sepa- 

 rated from it does not give the least vestige of colour when 

 we moisten it with an acid. In the process usually followed, 

 it frequently happens that the ochrey residue preserves 

 either &ome residue of blue which has not been attacked by 

 the alkali, or a mixture of prussiate of potash, and of ferru- 

 ginous alkaline carbonate, or even these three substances 

 confounded together. I shall now examine two of these 

 cases ; the third may easily be guessed at. 



If, for instance, we try an acid upon a residue well washed, 

 and which still retains some blue, this last will not be dis- 

 covered in pulverulent particles, except in proportion as the 

 acid will free it of yellow oxide. Between this oxide and 

 Prussian blue there is no particular chemical union, as has 

 been hitherto supposed; 1 at least there is nothing which po- 

 sitively indicates that the salino-metallic combination, which 

 we call prussiate ofiro?i, is, like so many others, susceptible 



Vol. 32. No. 127. Dec. 1808. Y of 



