1819.] _ Cyanogen and Hydrocyanic Acid. 437 
cyanide, the only change likely to be produced by the iron 
filings would be to a subcyanide, and we could not conceive 
how this body should resume its blue colour from the contact of 
water and air. But it is easy to conceive how the iron could 
deprive the hydrate of that metal of a portion of its oxygen, and 
thus change the compound into a protohydrocyanate of iron. 
It appears even that the oxide of iron formed during this opera- 
tion has not itself been separated, otherwise the prussian blue, 
when exposed to the air, would have assumed a greenish tint, 
which did not take place. Nor can we suppose that this white 
matter is a subhydrocyanate; because before such a compound 
could have been formed, hydrogen must have been disengaged, 
which was not the case. We might suppose indeed that the 
iron dividing the oxygen with that which enters into the compo- 
sition of prussian blue, had formed a combination which was a 
subprotohydrocyanate. But had this been the case, it would 
have assumed a green colour when left in contact with the air. 
“« This ought then to induce us to conclude that prussian blue 
is a hydrocyanate, and that the oxygen which it furnishes dur- 
ing its decomposition belongs to the hydrocyanic acid and to the 
iron. 
“ If we consider the great affinity of iron for oxygen, we shall 
scarcely believe that at the instant of the formation of prussian 
blue, in consequence of the contact of hydrocyanic acid with the 
hydrated oxide of iron, this last substance gives up its oxygen 
to the hydrogen of the acid, which itself strongly retains that 
substance. If we attend to the decomposition of water by iron 
and ‘by cyanogen itself, as has been shown above, we shall be 
still further from believing that prussian blue is a cyadide.” 
Action of Sulphuretted Hydrogen Gas on Cyanogen. 
By mixing together over mercury equal volumes of cyanogen 
and sulphuretted hydrogen, M. Vauquelin endeavoured to ascer- 
tain whether these two gases decomposed each other. The 
volume remaining the same, after an interval of some days, and 
no perceptible change having taken place, M. Vauquelin let u 
a quantity of water not sufficient to dissolve the whole of the 
cyanogen. Immediately on the contact of the water, the gases 
were rapidly absorbed ; the liquid assumed a yellow tint, which 
passed speedily to brown, and there remained merely a little 
azote proceeding from the decomposition of the cyanogen. The 
liquid had no perceptible smell, its taste was at first cooling, ' 
but became soon very bitter; and what was very remarkable, it 
did not sensibly redden litmus. When mixed with an acid solu, 
tion of sulphate of iron, it did not produce prussian blue ; but 
when potash was poured into the mixture, a yellowish-green 
precipitate fell, which, being redissolved by sulphuric acid, left 
a little prussian blue. ; 
The same liquor produced noacti n on acetate of lead. No 
