Metallic zinc is without influence on the system; and it is only in chemical combination that it becomes active. The effects of its preparations are closely analogous to those of the preparations of copper, though less energetic. In relation to their visible topical effects, they are, according to their degree of solubility and concentration, either mildly excitant and astringent, or actively irritant, or escharotic. Taken internally, they operate directly on the alimentary mucous membrane, and, through absorption, on the system at large. In reference to the former of these seats of their action, they are, in small doses, simply tonic and astringent; in larger, promptly and powerfully emetic; in still larger, highly irritant, and even escharotic, sometimes causing death by inflammation or destruction of the mucous membrane. This higher grade of action is exercised only by the soluble preparations. Upon the system at large, when given in small and repeated doses, they produce no sensible effect in health; and it is only by the relief or cure of certain morbid conditions, that they are inferred to exercise a tonic or corroborant influence on the nervous centres, analogous to that of silver and copper. That they are absorbed has been proved beyond question. After having been swallowed, they have been found in the secretions and the solid tissues. Dr. Michaelis, in his experiments on the lower animals, noticed that, though zinc was found in the urine, after the internal exhibition of the oxide, it was more largely eliminated with the bile (Arch. Gen., 4e ser., xxx. 481.) Taken in poisonous doses, together with the local injury of the primae vise, the preparations of zinc sometimes occasion symptoms evincive also of an action on the nervous centres, as coma, convulsions, and paralysis.

It has been a question, the decision of which is of considerable importance, whether the slow and continued introduction of zinc into the system is capable of materially deteriorating the health; in other words, of inducing a state of chronic poisoning, as the preparations of lead and some other metals are known to do. The general impunity under the long use of zinc as a medicine, and the comparatively little inconvenience experienced by. those engaged in the various manufactures or applications of the metal, would seem to determine this question in the negative; but facts have been brought forward, which appear to me to place beyond doubt the occasional action of the preparations of zinc in the way referred to. Thus, a patient who took twenty grains daily, for several months, of the oxide of zinc, for the cure of epilepsy, became pale, emaciated, and almost idiotic, with a furred tongue, constipated bowels, tumid abdomen, cold extremities, oedema of the lower limbs, dry, shrivelled skin, like parchment, and a slow, small, very feeble pulse; symptoms, however, which quickly disappeared upon the omission of the medicine, and the use of cathartics, with a tonic and supporting treatment. (Brit. and For. Med. Rev., July, 1838, p. 221.) Several men employed in barrelling oxide of zinc, and exposed for some days to an atmosphere loaded with the powder, experienced, from the beginning, loss of appetite, clammy taste in their mouths, and colic; and, after ten days, were attacked with vomiting, constipation, and violent colic, not unlike the affection produced by lead, and which, as that usually does, yielded to purging and opiates. (See Chem. Gaz., viii. 362.) In another instance, workmen exposed to the powder arising from beaten zinc, were affected with general depression, sore-throat, angina, ulceration of the tonsils, white pellicles on the gums, salivation, fetid breath, nausea, colic, and diarrhoea. The symptoms subsided, upon the abandonment of the occupation, in less than a week. (Ibid.) In these instances, it is obvious that large quantities of the powdered oxide must have been swallowed; and it was probably from this source that the symptoms proceeded. They were mainly such as result from a direct irritation of the alimentary canal; and, though it is probable that some of them arose from the absorption of the metal, they were of little importance. They moreover disappeared rapidly, on the removal of the cause, leaving no permanent effect behind. It is satisfactory that the evil from this cause seems so trivial, when compared with that from exposure to the influence of lead, for which zinc is in a course of rapid substitution as a material for painting. Abundant confirmation has recently been obtained of the morbid influence of zinc, largely introduced into the system, from observations made both in France and England, upon the effects of exposure to the fumes of the metal by the brass-founders. Workmen thus exposed, and especially those who have inhaled freely the fumes of oxide of zinc, arising from the combustion of this metal when heated in contact with the air, suffer much and often from a certain complication of symptoms, which, from an imperfect resemblance to intermittent fever, has received among them the name of brass-founder's ague. This subject has been carefully investigated by Dr. E. H. Greenhow, of London; and an essay containing the results has been published in the Medico-chirurgical Transactions (a.d. 1862, vol. xlv., p. 177). As stated by him, there is at first a sense of uneasiness or weariness, and of constriction of the chest, followed near bedtime by shivering and coldness, which ends in a hot stage, often attended with headache and vomiting, and invariably succeeded by profuse sweating. The patient is better the next day, but on fresh exposure is liable to fresh attacks, which, however, are quite irregular in their recurrence, and thus differ essentially from the genuine ague. Indeed, a febrile paroxysm, with its precursory nervous disorder, its cold, hot, and sweating stages, seems to have been developed by the presence in the blood of an absorbed poison, acting especially on the nervous centres. It is said that men engaged in this business are seldom long-lived, and are carried off in the end by the development of a pectoral disease called asthma, which, however, according to Dr. Greenhow, is chronic bronchitis. The symptoms might also be ascribed to the vapours of the copper employed; but, independently of the fact that the copper gives off vapours with much greater difficulty, this metal does not cause the phenomena in the absence of zinc.

In poisoning from large quantities of the soluble salts of zinc, the treatment should consist of the use of magnesia or one of the alkalies as an antidote, of free dilution with demulcent drinks, of opium to quiet irritation of the stomach and bowels, and of measures to combat inflammation corresponding with the exigencies of each particular case.

Of the therapeutic application of zinc it will be sufficient to treat under its several preparations. Of these, I would here observe, that, for internal use, all might well be spared except the sulphate and oxide, from which every curative effect can probably be obtained which the others are capable of producing.