xxvi _ Historical Sketch of the Physical Sciences, 1818. 
changes upon cyanogen; but much more rapidly than pure 
water, and the new acids evolved, instead of combining with 
. ammonia, unite with the potash itself, while the ammonia is 
' disengaged. 
When water impregnated with cyanogen is digested over 
peroxide of mercury, its peculiar odour soon disappears, and a 
portion of the oxide is dissolved. When the solution is concen- 
trated in a retort, carbonate of ammonia passes into the receiver, 
and two sets of crystals are deposited. ‘The first set consists of 
cyanide of mercury. The other crystals differ in their shape ; 
but whether they consist of cyanic acid and oxide of mercury 
could not be ascertained. Hydrocyanic acid is also present in 
the liquid. 
Hydrocyanic acid when placed in contact with the perhydrate 
of copper unites with it, and forms a yellowish-green compound, 
which crystallizes in small grains, and when washed in boiling 
water becomes white. The red matter which is formed by 
dropping common prussiate of potash into a solution of copper 
is in Vauquelin’s opinion a hydrate. It acquires a green colour 
when treated with liquid ammonia. This he considers as its true 
colour when deprived of water. 
From the observations contained in this paper, it would appear 
that Vauquelin has no knowledge of the ferro-chyazic acid of 
Porrett. Many of his conclusions are erroneous, from his not 
having attended to the difference between the hydrocyanic and 
the ferro-chyazic, two very distinct substances, which he seems 
to me to have always confounded together. Thus one of the 
most prominent parts of the paper consists in a set of experi- 
ments to ascertain whether prussian blue be a cyanide or a 
hydrocyanate of iron; he concludes from them that it is a hydro- 
cyanate. It does not seem to have occurred to him that it may 
likewise be a ferrocyanate of iron, which imdeed is the most 
likely opinion of all. 
Finally, M. Vauquelin has shown by his experiments that 
when the cyanide of potash comes in contact with water, there is 
always formed a quantity of carbonate of ammonia. This fact 
deserves the attention of the manufacturers of prussian blue.— 
(Ann. de Chim. et Phys. 1x. 113.) 
¢ 
Ill. ACIDIFIABLE COMBUSTIBLES. 
The acidifiable combustibles have been recently enriched with 
a new substance, I mean selentuwm—a substance detected by 
Berzelius, and approaching nearest to sulphur in its properties, 
though it differs in many respects from this combustible. I gave 
a sketch of its characters in the History of the Chemical 
Sciences for the preceding year, But the publication of Berze- 
lius’s experiments on it which has since taken place, will enable 
me to add several important particulars with which at that time 
we were unacquainted. Some few additions have been made to 
