_Hydro-cyanic Acid._ _Prussic Acid._
This peculiar acid exists in a great variety of native combinations in
the vegetable kingdom,[341] and imparts to them certain properties which
have been long known, and esteemed in medicine. It is, however, only
lately, that it has been administered in its simple but diluted form. As
few practitioners will choose to prepare the acid, it seems unnecessary
in the present work to dwell upon the merits of the different processes
which have been proposed for its preparation; for a full account of
them, as well as for other details of importance, the practitioner is
advised to consult a work by Dr. Granville, entitled “_An Historical and
Practical Treatise on the use of Prussic Acid. Second Edition. London,
1820._”
QUALITIES. A colourless transparent liquid, although it occasionally
exhibits a yellow tinge; _Odour_ like that of bitter almonds; _Taste_
bitterish and peculiar: these properties however are soon lost by
exposure to air and light, and the acid undergoes spontaneous
decomposition. CHEMICAL COMPOSITION. The true nature of _Prussic_ acid
was not ascertained until 1815, when Gay Lussac presented to the Royal
Institute of France, a memoir which at once developed its real chemical
constitution; and it is now admitted to consist of a peculiar gaseous
and highly inflammable compound of carbon and nitrogen, to which the
name _Cyanogene_ has been assigned, and of hydrogen; the latter body
acting as the acidifying principle, whence the term _Hydro-cyanic_ acid
is well contrived to express its composition. The medicinal, or
_diluted_ acid, however, contains but a small proportion of this
concentrated compound; according to M. Majendie, one part of the acid of
Gay Lussac and eight parts and a half of water, by weight, or one part
of acid with six times its volume of water, constitute the preparation
which should be used in medicine; and which, to avoid the possibility of
mistake, ought always to be prescribed as the Acidum Hydro-cyanicum
_dilutum_, and is, in fact, the _Prussic acid_ of Scheele. Dr. Ure, who
has lately taken considerable pains upon this subject, has constructed a
table exhibiting the relations between the specific gravities, and
quantities of real acid, in preparations of different strength; from
these experiments it would appear that an acid of specific gravity 0·996
or 0·997 is such as is usually prescribed in medicine.[342] MEDICAL
USES. In a sufficient dose, hydro-cyanic acid instantly destroys life by
extinguishing the nervous energy of the body;[343] but it has at the
same time been observed that animals submitted to its action would often
continue to breathe for several hours freely, and to circulate their
blood, although no trace of sensibility or muscular contractility could
be found after its application. This remarkable property of
extinguishing the general sensibility, without any ostensible injury to
respiration and circulation, naturally led to a belief that the
hydro-cyanic acid, or prussic acid, might be advantageously used in
cases of excessive sensibility and irritation, particularly when these
two morbid states are likely to affect either the respiratory organs or
the circulation generally. This kind of analogical reasoning, it is
said, induced Professor Brera, ten years ago, to administer it in cases
of high pulmonary and other inflammations, in doses of four drops twice
a day; when, as we are told, the violence of the disease was quickly
subdued. The remedy, however, does not appear to have excited much
attention, until after the first essay of Dr. Majendie, who deserves
whatever credit may belong to its introduction. Seven years of trial
have elapsed, and the general sense of the medical profession with
respect to its utility may now be collected. As a palliative in certain
spasmodic coughs, there is reason for supposing that it _may sometimes_
be useful, but in that species of pulmonary irritation for which it was
at first so greatly extolled, I will venture to assert that it is far
inferior in efficacy to well directed doses of _Conium_. But there is
another class of diseases in which its exhibition is said to prove
useful, in dyspeptic affections attended with heartburn; where it is
supposed to be capable of reducing the morbid irritability of the
stomach, and thereby of enabling the juices of that organ to be more
slowly secreted and of a more healthy character. Dr. Elliotson has
published the result of his treatment of stomach complaints with this
medicinal agent, and would appear to appropriate to himself the merit of
originating the practice, a claim which Mr. Thomson, in the third
edition of his Dispensatory, refuses to concede.[344] As a local remedy,
prussic acid has also received no small share of commendation, and it
has been said that it is the only application that can be depended upon
for allaying the cutaneous irritation so frequently attendant upon
certain impetiginous affections. It must, however, be confessed that
this medicine is rapidly declining in popularity.[345] Any prejudice
raised against it, upon the ground of its poisonous activity in large
doses, is too absurd to be believed; the knife and the caustic are
unquestionably powerful, and may therefore become dangerous instruments;
but who ever blames the surgeon for employing a sharp knife or an active
caustic, seeing that both are to be directed by his eye, and guided by
his hand? FORMS OF EXHIBITION. It may be conveniently administered in
any liquid vehicle, as distilled water, camphor mixture, or in some
vegetable infusion. See _Supplementary Formulæ_, 1, 2, 3. A question has
lately arisen whether the effects of the prussic acid might not be more
conveniently ensured by the administration of some vegetable[346] in
which it exists as a native ingredient; a company of associated
Physicians, Surgeons, and Naturalists at Florence, have accordingly
expressed their joint opinion, that the essential oil of the _Prunus
Lauro Cerasus_ is to be preferred in medical practice to all other
preparations which contain the hydro-cyanic acid; for, say they, unlike
the distilled water of the plant, and pure prussic acid, it contains the
same proportion of active matter, and of the same power, whether
recently prepared or not; whether made in one place or another; or
whether it has been exposed or not to the air, light, or heat. They are
also of opinion that olive oil forms the best vehicle for its exhibition
in the proportion of one ounce to twelve drops of the essential oil.
Other practitioners again prefer _Laurel Water_, made by distilling two
drachms of the fresh leaves chopped, with four ounces of water,
recommitting the distilled water twice afterwards on the same quantity
of fresh leaves, and making ultimately four ounces of the menstruum, of
which from ♏︎xxx to fʒj every six hours may be given until a sedative
effect is produced. See _Oleum Amygdalæ Amaræ_. INCOMPATIBLE SUBSTANCES.
Hydro-cyanic acid is decomposed by most of the _oxydes_ usually employed
in medicine, particularly by those of _Mercury_ and _Antimony_. The
alkalies do not appear to diminish its efficacy. _Nitrate of Silver_,
and the salts of iron occasion precipitates; nor ought the _sulphurets_,
the _mineral acids_, or _chlorine_ to enter with it into prescriptions.
DOSE. Of the medicinal, or _diluted_ hydro-cyanic acid, ♏︎ij,–viij.
There is however considerable difficulty with regard to the strength of
the dilute acid employed in medicine, since the density is a criterion
of greater nicety than can be conveniently used by the majority of
practitioners; in fact, as Dr. Ure has observed, the liquid at 0·996,
contains about double the quantity of real acid, which it does at 0·998.
Dr. Ure has accordingly proposed another test of the strength of this
powerful and dangerous medicine, which is not only easier in use, but
more delicate in its indications;[347] it is as follows. To 100 grains,
or any other convenient quantity of the prussic acid, contained in a
small phial, add in succession, small quantities of the peroxide of
mercury, (the common _red precipitate_ of the shops) in fine powder,
till it ceases to be dissolved on agitation. The weight of the red
precipitate taken up, being divided by four, gives a quotient
representing the quantity of real prussic acid present. By weighing out
before hand, on a piece of paper, or a watch glass, forty or fifty
grains of the peroxide, the residual weight of it shews at once the
quantity expended. The operation may always be completed in five
minutes, for the red precipitate dissolves as rapidly in the dilute
prussic acid, with the aid of slight agitation, as sugar dissolves in
water. ADULTERATIONS. If, says Dr. Ure, the presence of muriatic acid be
suspected, then the specific gravity of the liquid compared with the
gravity of the peroxide dissolved, will shew how far the suspicion is
well founded; thus if 100 grains of acid, specific gravity 0·996,
dissolve more than 12 grains of the red precipitate, we may be sure that
the liquid has been contaminated with muriatic acid. _Nitrate of
Silver_, in common cases, so valuable a reagent for muriatic acid, is
unfortunately of little use here, for it gives with prussic acid, a
flocculent white precipitate, soluble in water of ammonia, and insoluble
in nitric acid, which may easily be mistaken by common observers, for
the _chloride_ of that metal. But the difference in the volatility of
prussiate and muriate of ammonia may be had recourse to with advantage;
the former exhaling at a very gentle heat, the latter requiring a
subliming temperature of about 300° _Fah._ After adding ammonia in
slight excess to the prussic acid, if we evaporate to dryness at a heat
of 212°, we may infer from the residuary sal ammoniac, the quantity of
muriatic acid present.
ANTIDOTES. To counteract the poisonous effect of prussic acid, Orfila
recommends, after full vomiting has been excited, the exhibition of
three or four spoonsful of oil of turpentine, in the infusion of coffee,
at intervals of half an hour. M. Virey conceives that sulphate of iron
in solution is the best antidote, he having observed that the salt
restored a cow that was nearly killed by the essential oil of bitter
almonds. When an overdose has been taken, hot brandy and water, and the
ammoniated tincture of iron are recommended by Mr. Thomson; on the
former I should rely with much greater confidence than upon the latter
antidote, or, in other words, it is from vital agents, counteracting its
sedative influence, rather than from chemical substances, changing its
composition, that we can expect any benefit upon such an occasion. For
the chemical processes by which the presence of this acid may be
ascertained, the reader may consult my work on Medical Jurisprudence,
vol. 2, p. 408.