46 
BRITISH ASSOCIATION.— 1835 . 
those to which cyanogen gives rise when combining with the same 
bases. The method proposed by Dr. Geoghegan is founded on the 
property which the double salt of the iodide of potassium and bi¬ 
cyanide of mercury possesses of being decomposed by acids, and 
then producing biniodide of mercury. This compound, which has 
been analysed by Liebig, and subsequently by Dr. Apjohn, is easily 
prepared by mixing, in the proportion of atom and atom, the iodide 
of potassium and bicyanide of mercury, each dissolved in a small 
quantity of hot water. After a short time silvery scales (resem¬ 
bling acetate of mercury) are formed, which constitute the salt in 
question. The circumstance of this salt being decomposed by all 
the ordinary acids, would appear to show that it is not capable of 
demonstrating the presence of muriatic acid in particular ; but as 
the only other impurities likely to be present in the hydrocyanic 
acid are sulphuric and tartaric acids, if the appropriate tests of 
these latter do not indicate their existence, then the formation of 
biniodide of mercury on the addition of a crystalline scale, or solu¬ 
tion of the double-salt above mentioned, may be considered as fur¬ 
nishing conclusive evidence of the presence of muriatic acid. It 
may be also stated, that the only hydrocyanic acid likely to con¬ 
tain sulphuric-—that prepared from the ferrocyanide of potassium— 
can be generally recognised, as to the source from whence derived, 
by its possessing a slight bluish or bluish-green tinge, which is 
quite distinctive. The mode of detecting the presence of muriatic 
acid above detailed has the advantage over those usually em¬ 
ployed, of being very readily applied, and the formation of the re¬ 
agent is perfectly simple; it is capable of detecting l-4500th part 
of the acid: if no change of colour ensue on the addition of the 
salt, we may conclude that the specimen of hydrocyanic acid con¬ 
tains no impurity which can interfere with the subsequent estima¬ 
tion of its strength. This method, however, is inapplicable to the 
alcoholized acid of Germany, as the biniodide is soluble in spirit, 
yielding a colourless solution. If the presence of muriatic acid 
have been ascertained, its neutralization can be readily effected by 
the addition of successive small portions of precipitated carbonate 
of lime, as long as any is dissolved ; when free, muriatic acid has 
been got rid of, and not till then can the estimate of the strength 
of the specimen under examination be proceeded in with any hope 
of a correct result. The method of Dr. Ure for effecting this lat¬ 
ter end is sufficiently correct for ordinary purposes, if we substi¬ 
tute for the red precipitate which he employs, pure peroxide of 
mercury; as, independent of the presence of minium and other im¬ 
purities, red precipitate is seldom, if ever, free from pernitrate of 
mercury, if perfect accuracy be desirable, the best method, and 
probably as simple a one as that just alluded to, is the formation of 
cyanide of silver by the addition of the nitrate of that metal. 
