I 
Hydrocyanic Acid fyc. 168 
6ipitated is as 3: 1, and not as 6: 1 ; by (c.) that the relation 
of the cyanogen disengaged as hydrocyanic acid is to that in 
the precipitate as 1; 1,-and not as 12: 9. And the quantity of 
yellow salt produced in (a.) serves to confirm both the above 
results. 
The theory of the subsequent conversion of the salt into 
Prussian blue, by moistening it with dilute sulphuric acid and 
exposing it to air, is consequently unknown. I have not yet 
examined the precise change which takes place, with suffi- 
cient care to give an opinion : that potassa is dissolved out, 
and that the action of free oxygen is essential to the change, 
is certain. 
(5.) Had I examined Gay Lussac's paper before I began 
my experiments, his high authority would have made me con- 
sider any further experiments on this subject as useless ; but 
as I had finished the experiments marked Nos. 1 and 2, be- 
fore I saw his paper, I was induced to repeat my experi- 
ments with redoubled care : hence the series No. 3, and 
hence their nearer approach to the calculated numbers. I 
must therefore conclude that M. Gay Lussac has operated on 
the salt obtained by the action of concentrated sulphuric acid 
on the crystals. The change in that case, according to 
Thompson, is so complicated that sulphurous gas, ammonia, 
carbonic oxide, azote, are given off. I doubt if any definite 
conclusions can be drawn from it. 
(6.) The best proportions, therefore, of the ferrocyanuret 
of potassium and sulphuric acid to be used when we want 
hydrocyanic acid are as follows. To every 212.47 grains of 
the crystals dissolved in about 2 fluid ounces of water, add 
so much dilute sulphuric acid as shall contain 120 grains of 
real acid, and by conducting the distillation carefully, 41 
grains of hydrocyanic acid pass off, and that I find with the 
first third of the water : of course the water must be put into 
the receiver and kept very cold. But no process for pro- 
curing a dilute solution of hydrocyanic acid, in which dis- 
tillation or filtration is had recourse to, will yield an acid of 
uniform strength, however carefully the process may be con- 
ducted, not even, as I have proved, if the receiver be sur- 
