"Take of cyanuret of mercury1, one ounce; muriatic acid, seven fluid drachms; water, eight fluid ounces. Distil from a glass retort into a cool receiver eight fluid ounces; and preserve it in a well-stopped bottle, in a cool, obscure place.

"The sp. grav. should be 998 to distilled water as 1000."

This acid is found in many vegetable productions; such as the bark of the prunus padas, or bird-cherry; the leaves of the prunus lauro-cerasus, and of the peach and nectarine trees; in bitter almonds, and the kernels of many fruits; but, for the purposes of medicine, it is artificially prepared.

In the above process of the London College, the ferrocyanide of potassium is decomposed by the sulphuric acid.

When two equivalents of the ferrocyanide are heated with six of sulphuric acid, three equivalents of the cyanide of potassium which it contains are decomposed, and also three equivalents of water; the oxygen of the latter combining with the potassium of the cyanide converts it into potassa, which unites with the six equivalents of sulphuric acid, and forms a bisulphate of potassa; whilst the hydrogen unites with the three equivalents of cyanogen which are set free, and forms with them three equivalents of hydrocyanic acid. According to Mr. Everitt, whose explanation of the process this is, there remain one equivalent of cyanide of potassium, two equivalents of cyanide of iron, and these combining form what he terms a yellow salt, the constitution of which is the opposite of that of the ferrocyanide of potassium, in reference to the proportion of the cyanides.

1 For forming the cyanuret of mercury, the Prussian blue should be purified by digesting it in diluted hydrochloric acid; being washed and dried, add to 8 parts of it 11 of peroxide of mercury, and boil them together in water; a colourless solution is obtained, which, in evaporation, yields pure white crystals of cyanuret of mercury even to the last drop.

I find that this yellow salt is only obtained when the process is stopped at the point directed in the pharmacopoeia: but when it is carried on to extreme dryness, an additional quantity of the hydrocyanic acid is procured, and the residue consists of bisulphate of potassa, and Prussian blue.

In the process, as ordered by the pharmacopoeia, the acid is always obtained of a pale green colour, and requires redistillation to free it from this tint: but the acid procured by continuing the process to dryness is perfectly colourless. There is, therefore, waste in the process of the pharmacopoeia: the whole of the acid should be brought over and redistilled, and then diluted to the requisite degree of strength.

In the Dublin process, the bicyanide of mercury is decomposed by the heat, aided by the hydrochloric acid; and the cyanogen uniting with the hydrogen of the water or of the hydrochloric acid, forms hydrocyanic acid vapours, which, coming over in conjunction with the watery vapour into the receiver, constitute the fluid hydrocyanic acid. It generally contains a small portion of hydrochloric acid, which may be separated by redistilling, after the acid has been mixed with a little chalk: the hydrochloric acid is thus left in the retort in combination with the lime of the chalk. The retort employed should be very large; and the acid made in small quantities at a time.

Various other processes may be employed for furnishing this acid. 1. Dissolve 3 viij. 1/4 of cyanide of potassium in 3 c. of distilled water, and add 3 xviij. 3/4 of tartaric acid dissolved in 3 xx. of distilled water. In this case, the oxygen which is required to be added to the potassium to form it into potassa, and the hydrogen to the cyanogen to form the hydrocyanic acid, are obtained from the decomposition of the water. The insoluble residue is bitartrate of potassa. 2. Mr. Laming proposes to add alcohol, instead of a portion of the water, to aid the precipitation of the bitartrate, and to preserve the acid from decomposition. 3. Pass a stream of sulphuretted hydrogen through a given quantity of ferro-cyanide of potassium dissolved in a given quantity of water; an insoluble sulphuret of mercury, and a limpid solution of hydrocyanic acid are formed, which can be easily separated by simple decantation. It is necessary, however, to agitate the acid with carbonate of lead, in order to remove any excess of sulphuretted hydrogen which it may hold in solution.

Physical and chemical properties. - Hydrocyanic acid, prepared by the above-described processes, is a colourless, transparent liquid, with an odour which is supposed to be like that of bitter almonds; but it differs materially from that, and is peculiar to itself. It is at first cooling, bland, and sweetish to the taste, but ultimately it impresses a pungent acrimony on the palate. It is very volatile. Its specific gravity is 0.998. It combines in every proportion both with water and alcohol.

It is, when pure, readily decomposed by a high temperature and by light; being resolved into carbonic acid, ammonia, and carbureted hydrogen gas, which are dissipated, and leave behind a carbonaceous deposit. It should, therefore, be kept in an opaque stoppered bottle. It is very inflammable, burning with a blue flame. According to Gay Lussac, it consists of a peculiar base, which he has named Cyanogen, acidified by hydrogen. Cyanogen is a compound, consisting of two eq. of carbon, = 12.24 + 1 of nitrogen, - 14.15, making the equivalent = 26.39, or a bicarburet of nitrogen. But the acid employed in medicine contains one part only of the acid referred to by Gay Lussac, and eight parts and a half of water. The composition of the strong acid is 1.8054 of cyanogen and 0.0694 of hydrogen, or one eq. of cyanogen =26.39 + 1 of hydrogen = 1 equiv. 27.39.

1 Brera's work is entitled "Prospetti dei risultamenti ottenuti nella Clinica