Protochloride Of Mercury

Subchloride of Mercury.

Calomel is prepared by first forming a sulphate of the deutoxide of mercury, by boiling sulphuric acid and the metal together to dryness. This is then rubbed with a quantity of mercury equal to that which it contains, and afterwards with chloride of sodium in a certain proportion, and finally submitted to sublimation. it may be supposed that the mercury forms, with the deuto-sulphate first obtained, a sulphate of the protoxide, which reacts with the chloride of sodium, so as to form sulphate of soda, which remains, and protochloride of mercury, which is sublimed. The vapours condense into a heavy crystalline cake, which is to be finely powdered, and washed thoroughly with boiling distilled water, in order to separate some corrosive sublimate, which is always formed, and sublimes with the calomel. This impurity is known to have been completely removed, when the washings do not yield a white precipitate with solution of ammonia.

Properties

As above prepared, calomel is a heavy, yellowish-white or ivory-coloured powder, inodorous, tasteless, and insoluble in water, alcohol, and ether. it is darkened by exposure to light, but otherwise is unalterable in the air. it is completely dissipated by heat. According to the view which considers the combining number of mercury as 202, it consists of one equivalent of the metal and one of chlorine.

Incompatibles. Calomel is decomposed by the alkalies and alkaline earths, with the formation of the black oxide of mercury. The alkaline carbonates, soaps, and soluble sulphurets also decompose it; and the same is said to be the case with several of the metals. Nitromuriatic acid probably converts it into corrosive sublimate; and I have been informed of a case, in which these medicines were jointly administered, with the apparent effect of causing excessive vomiting and purging, which ended fatally. Alkaline chlorides have the same effect, but much more slowly. Chloride of ammonium, however, or muriate of ammonia acts with considerable energy; and a fatal case is on record, in which death was supposed to have resulted from the use of this salt simultaneously with calomel. Hydrocyanic acid also decomposes calomel, forming corrosive sublimate and deutocyanide of mercury; and should not, therefore, be given along with it. For the same reason, all the natural products containing hydrocyanic acid, such as oil of bitter almonds, bitter almond water, cherry-laurel water, syrup of orgeat, infusion of wild-cherry bark, etc., should be used with caution, if at all, when calomel is administered.

Effects on the System

Calomel is of itself scarcely irritant; but, in consequence of the change it undergoes when in contact with the liquids of the body, and of course with the mucous surfaces, or those of ulcers or abrasions, it becomes so in a greater or less degree, corresponding with the rapidity of the change. The nature of this alteration has been already referred to. According to Mialhe, whose views, to say the least, are extremely plausible, whenever calomel comes in contact with the animal liquids, containing an alkaline chloride, the chloride of sodium, for example, it is converted partially into corrosive sublimate; one part of the protochloride giving up its chlorine to the other so as to form deutochloride with reduction of a portion of the metal; and the salt thus formed combining with the alkaline chloride which gives it stability. it is thus the corrosive sublimate, or the compound of this with the alkaline chloride that is the real agent, whenever calomel operates in any manner, either locally or upon the system. in this way, peculiarities in the operation of the medicine, before unaccountable, are explained without difficulty.

Calomel produces on the system all the effects, already fully described, which characterize the mercurial preparations; and, on the whole, is the best of them for internal use, being at the same time, when properly guarded, in general mild in its operation, and yet certain and effective. in large doses, it is purgative and anthelmintic; and, in still larger doses, it is thought, with a moderate purgative effect, to have a peculiar sedative influence upon the alimentary mucous membrane, for which it has been much used in cholera and dysentery, with great asserted advantage. in these several relations, I shall treat of the medicine among the cathartics and anthelmintics.

Many instances have been recorded, in which calomel has operated poisonously; in some destroying life by excessive salivation, in others, by a powerful irritant influence on the alimentary mucous membrane. Yet the cases are infinitely more numerous, in which it has been administered in enormous doses, often as much as drachms at a time, and, taking the aggregate of the doses given at short intervals, amounting sometimes to ounces, or even a pound, within a very few days, without any observable physiological results whatever, except some depressing sensations in the abdomen, moderate purgation, or a slight salivation, and not always these. I have heard of a practitioner, who was in the habit of giving calomel by the teaspoonful, in certain violent cases of disease; and it is quite certain that many of his patients escaped without injury. With our present lights on the subject, it is not difficult to explain these discrepancies. Merely as calomel, the remedy is bland and probably inert. By conversion, however, into corrosive sublimate, it acquires activity; and this activity is just in proportion to the quantity of the soluble compound produced. But the change is due to the alkaline chlorides present, and is proportionate to their amount. if there should be little of them, their power will soon be exhausted, and the calomel will produce but moderate effects, the greater portion remaining unchanged and inert in the bowels; and, so far as regards immediate effects, it differs little how much this inert remainder may amount to, whether to grains or to drachms. The result is the same whether the chlorides pre-exist in the primae viae, or are introduced. The inference from all this is that, when Calomel operates violently, either as an irritant or sialagogue, it meets with an unusual proportion of the alkaline chlorides in the primae viae, either secreted or introduced; and, when comparatively inert, that it is so because it encounters little or none of these salts. Now, whether this explanation is admitted or not in its precise terms, it is at least highly probable that the different results obtained from the medicine are owing to its different degree of solubility, under varying circumstances, in the liquids of the alimentary canal.