•216 
DR. GLADSTONE ON CIRCUMSTANCES MODIFYING 
till of the same volume as the first portion. It was far deeper in colour, indicating 
evidently that the insolubility of sulphate of potash in alcohol had removed it out of 
the sphere of action, and had caused a much larger proportion of ferric sulphocyanide 
to be formed than would otherwise have been produced. Only a small quantity of 
alcohol was employed, and it was mixed with a large amount of water in order to ob- 
viate as much as possible the objection that the same amount of ferric sulphocyanide 
might appear darker in alcoholic than in aqueous solution, M^hich is indeed the fact. 
II. Another red solution was prepared by mixing sulphocyanide of potassium and 
ferric sulphate, and it was divided into two equal portions. To one of these hydro- 
chloric acid was added, which of course reduced the colour somewhat. To each was 
then added an equal portion of neutral phosphate of soda. The acid solution remained 
red, though paler than before ; the neutral solution became colourless, and turbid 
from the formation of a floeculent precipitate of ferric phosphate. That the insolu- 
bility of this salt in the neutral solution was the cause of the complete combination 
of the oxide of iron with the phosphoric acid, was further elucidated by adding phos- 
phoric acid to the colourless mixture, which restored a faint red tint to the solution, 
doubtless because it had set free some of the sulphuric acid, which, redissolving the 
ferric phosphate, allowed of the formation of a small amount of the red sulpho- 
cyanide. 
III. A mixture of three parts of ferric citrate, and four of ferrocyanide of potas- 
sium was prepared, and divided into two equal parts. To the one there was added 
a few drops of hydrochloric acid, to the other a few drops of oxalic acid. In the 
one case, the ferric ferrocyanide, being insoluble in hydrochloric acid, was precipi- 
tated, leaving no trace of iron in the solution ; in the other case there was a blue 
solution, but the whole of the iron was not in the condition of ferric ferrocyanide, 
for the addition of more prussiate of potash caused it to become bluer. That this 
was due, not to the affinity of the oxalic acid for the ferric oxide, but to that of the 
citric acid, will be evident from the fact ascertained by the previous experiments on 
the ferrocyanide, that this result would not have been obtained had the nitrate been 
employed. 
IV. It is well known that if hydrosulphuric acid gas be passed through a solution 
of arsenious acid in water containing a mineral acid a perfect separation of the yellow 
sulphuret takes place, but if there be no free acid the solution remains yellow. The 
question suggested itself — Is the whole of the arsenious and hydrosulphuric acids 
converted in the latter case into sulphide of arsenic and water ? In order to ascertain 
this, a stream of sulphuretted hydrogen was passed through a solution of arsenious 
acid in water till it Avas saturated, then it was allowed to stand awhile for the excess 
of sulphuretted hydrogen to be dissipated, and hydrochloric acid was added. The 
yellow sulphide was of course precipitated, but on repeated trials I always found 
more or less arsenious aeid still remaining in the solution. 
77/e testimony of volatilization . — The argument that has been employed in the 
