Much has been said of the poisonous effects of calomel. in numerous instances, even in moderate doses, it is asserted by some writers to have produced the most alarming symptoms, and sometimes to have occasioned death; and, in very large doses, has been accused of exercising the most murderous powers. Yet authorities more numerous, and quite as authentic, can be adduced in favour of its harm-lessness in large quantities; and, in this country, every physician knows that it has been given in doses of a scruple, a drachm, several drachms, an ounce even, by adventurous practitioners, without obvious injury. I have been told of one case of acute disease, in the course of which a pound was employed, without any reason to suppose that it hastened the fatal issue. Cannot these discrepancies be reconciled ? The reader who has attentively perused the preceding observations upon the effects and mode of action of calomel, is I think furnished with a clue which may lead him out of this labyrinth of contradictions. I have not the least doubt, that much of what has been said on the injurious effects of calomel has been the result of partial or prejudiced observation, often ascribing the consequences of disease, or merely accidental coincidences from other causes, to the use of this remedy. I cannot but think so; because, in the course of a practice extending through a period of more than forty years, during which I must have prescribed calomel, and seen it prescribed to thousands, I cannot recall a single instance, within my own personal experience or observation, not only of any fatal effect, but of any serious permanent injury from its use. Yet nothing is more certain than that fatal effects have sometimes been produced by it; that, in a large number of cases, it has brought patients into states of greater or less danger; and that, in a few of these, there has been left behind, after recovery, serious deformity of the face, and perhaps other permanent injury. it will be understood that I am here considering calomel as employed in purgative doses, and without a view to its systemic effects, of which enough has been said under mercury as an alterative.

When calomel has produced dangerous or fatal irritation or inflammation of the alimentary mucous membrane, the result may be ascribed either 1. to its containing a considerable proportion of corrosive sublimate, 2: to its being administered shortly after, or at the same time with muriate of ammonia, nitromuriatic acid, or other agent capable of rendering it soluble in the gastric liquids, or 3. to the accidental presence of a large and abnormal quantity, in the stomach and bowels, of the alkaline chlorides, under circumstances favourable to the rapid chemical change in the calomel, already referred to. it is obvious that it is the last only of these sources of injury that may not be avoided with due care; and even this is so rare an event, if we are to decide upon the grounds of experience, that no injury need be apprehended, with a moderate observance of the ordinary rules of prudence. The change is effected slowly by these agents as they exist in the stomach and bowels; and, if proper attention is paid to the warnings given in any particular case, all danger may be avoided by free purgation with castor oil or sulphate of magnesia. I do not here consider the danger of hypercatharsis, to which every patient of an ignorant or reckless practitioner is liable, from an abuse of this, as of any other energetic purgative.

Another danger from calomel is that, instead of operating upon the bowels, it may violently affect the system, and operate destructively through excessive inflammation of the mouth and neighbouring parts, or great disturbance of the vital functions. This is, I believe, a much more serious source of evil than the preceding. No practitioner who uses calomel at all as a purgative is absolutely secure against these results; for the susceptibility in some individuals is so great, that the smallest quantity ever used with a view to cathartic effect, may induce in them severe ptyalism. Yet, with due caution, not only in the use of the medicine, but in making inquiries as to the constitutional peculiarities of individuals in this respect, the danger is so diminished as to be almost infinitely small; and it should not be allowed to weigh; for a moment, against the great benefits to be derived from the medicine.

The question now suggests itself, whether the use of calomel, in the large doses referred to, is compatible with this due caution. I think it is not. Nothing is more true than that they are often given with entire impunity, especially in those diseases of the stomach and bowels for which they have been most frequently prescribed. The reason of this impunity is, obviously, that only a small proportion of the calomel really acts, the other being entirely inert as regards any dynamic operation, and equivalent only to as much inert powder of any other kind, having the same physical qualities. The portion of the calomel which operates is only that which is rendered soluble, and this is limited by the amount in the stomach and bowels of the reagents calculated to render it soluble. Now, as a general rule, there is not enough of these to do more than to enable a full dose of calomel to act on the bowels vigorously; and, as a general rule, therefore, there is no danger of serious gastro-enteritis. Moreover, still speaking in a general way, the change in the calomel which fits it for absorption is so slow, as not to endanger serious mischief, even though the calomel should not happen to be rapidly carried off by purgation. But there are exceptions to these general rules. There may happen to be chemical agents enough in the bowels to react on much more than the ordinary amount of calomel, and convert it into a powerful irritant; and there may happen also to be an excessive susceptibility to the constitutional impression. There will, therefore, be occasional danger, from these large doses, of serious gastro-enteritis, or hyperemesis and hypercatharsis, or of a dangerous action on the mouth or the vital functions. This risk should never be incurred, unless for some highly important, and otherwise unattainable end; and it is extremely doubtful, in my mind, whether disease offers to us any occasion of this kind. induced by these considerations, I have never employed the excessive doses of calomel recommended by some writers, and would not advise their employment under any circumstances by others. There will be occasion to say more upon this point, when treating of the use of large doses of calomel in dysentery and cholera.