90 HISTORY OP PRUSSIATES. 



Action of Alkalis. 

 Treated with The blue passed through caustic potash leaves a residuum, 

 caustic potasii, w ]iich is nothing but red oxide blended with alumine. Its 

 colour is that of kermes, if the blue were of good quality; 

 on the contrary it is pale and earthy, if the blue were sur- 

 charged with alumine: so that we may form a pretty good 

 judgment of its nature by the colour of its residuum, 

 may T>c de- Acids acquire no colour from the residuum properly 



pnved of at! its -washed ; which shows, that Prussian blue may be deprived of 

 acid at once. „-.-., 



all its acid at a single operation ; but for this it must have 



been very finely pulverized, which is attended with some dif- 

 ficulty. If a few drops of alkali be added to water coloured 

 by the blue recently precipitated, it will be deprived of its 

 colour completely; and in this case the oxide separated from 

 it will notaiford the least trace of colour, when it is wetted 

 This seldom with an acid. In the process followed it frequently happens 

 that the ochry residuum retains either some remains of blue, 

 that have not been touched by the alkali, or a mixture of 

 prussiate of potash and ferruginous alkaline carbonate, or 

 all three of these blended together. I shall proceed to ex- 

 amine two of these cases, and it will be easy to form a judg- 

 ment of the third. 

 Blue left in the If for example an acid be applied to a well washed resi- 

 residuum. duum, which still retains Prussian blue, this blue will not 

 discover itself in pulverulent particles, but in proportion as 

 the acid frees it from yellow oxide. There is no parti- 

 cular chemical union between this oxide and Prussian blue, 

 as hitherto has been supposed ; at least we have no positive 

 indication, that the metallic salt, called prussiate of iron, is 

 susceptible, like so many others, of a maximum and mini- 

 mum state either of acid or of oxide ; and if the mixture of 

 yellow and blue, which these residuums sometimes offer us, 

 be not green, as might be expected, it is because the yellow 

 oxide always covers these remains of blue in very great ex- 

 cess ; at least I have never found the blue to be above one or 

 two hundredth parts. 

 Prussiate of I proceed to the second case. A residuum may contain 



feTrJinomal- no remains of blue, * f it were well pulverized, but it easily 

 k.-ilinecarbo- retains the two salts I have mentioned above. If an acid 

 ate » be then applied to it, each of them affords abundance of 



5 blue 



