1819.) Cyanogen and Hydrocyanic Acid. 435 
water is poured upon these substances, the purple-coloured body 
alone dissolves, and gives the liquid a fine purple colour. The 
substance which remains undissolved is prussian blue, which 
had been held in solution in the hydrocyanic acid. Some drops 
of chlorine let fall into this liquid change it to blue, and a 
greater quantity destroys the colour entirely. It is remarkable 
that potash poured into the liquid, thus deprived of its colour, 
occasions no precipitate whatever. 
_ © Chemists will not fail to remark from these experiments 
that hydrocyanic acid does not form prussian blue directly with 
iron ; but that on the addition of water (circumstances remaining 
the same) prussian blue is produced. 
«« They will remark likewise, that cyanogen united to water 
dissolves iron. This is confirmed by the mky taste which it 
acquires, by the disappearance of its colour, and by the residue 
which it leaves when evaporated; yet prussian blue is not 
formed. 
«« These first experiments seem already to show, that prussian 
blue is a hydrocyanate, not a cyanide. 
Action of Heat on Prussian Blue. 
“To complete our conviction of the nature of prussian blue, 
it appeared necessary to examine it with care; and in the first 
= I shall explain the phenomena which take place when it is 
dried. 
“ Tt took fire like pyrophorus, and continued to burn till it. was 
entirely destroyed, although the platinum vessel in which it was 
_ contained was removed from the fire. During the whole time 
that this combustion lasted, hydrocyanate of ammonia was dis- 
engaged, as was easily ascertained by the smell. The residue 
was red oxide of iron. 
“ The ammonia and hydrocyanic acid disengaged during the 
whole duration of the combustion of prussian blue, give a new 
support to the opinion above given, that this substance is a 
hydrocyanate of iron. 
“ Prussiate of iron purified by sulphuric acid and dried as 
much as possible was distilledin a retort. Soon after the opera- 
tion began, drops of water were seen condensed in the beak of 
the retort. Afterwards, when the heat had become stronger, a 
white vapour appeared, which condensed into needle-form 
crystals. The gas extricated during this operation was collected 
in four jars of the same size. The first portion, when mixed 
with a solution of potash, lost about a third of its volume. The 
two-thirds not absorbed burned with a blue-coloured flame, and 
the product of the combustion precipitated lime-water. The 
potash employed in this operation did not sensibly effervesce 
with acids; but it rendered lime-water slightly milky, and .it 
formed beautiful prussian blue with the acid sulphate of iron. 
This shows that the gas absorbed was chiefly hydrocyanic acid. 
252 
