* Some doubt is thrown upon the uniform occurrence of the chemical reaction here referred to, by an experiment of Professor Procter, performed at my request. A drop of solution of nitrate of silver was made to fall into a solution of chloride of sodium, and produced a precipitate; yet, after long contact of this precipitate with the supernatant solution, the latter, when filtered off. gave no evidence of the presence of silver when applied to bright copper, and touched with bright iron; and the taste was purely saline.

How far nitrate of silver would operate as a deadly poison, in excessive amount in the human subject, is left very much to inference; for few cases of acute and fatal poisoning from this substance have been recorded. Boerhaave mentions the case of a pharmaceutical student, who, having swallowed a portion of lunar caustic, died from gangrene of the first passages. But this is the only case of the kind that 1 have met with in the books. In the Bulletin Generate de Therap. (Sept. 1839), the case of a young man is related, who, after having taken a large quantity of the salt in solution, an ounce, according to his own account after recovery, was carried to the Hospital St Louis, in Paris, quite insensible in all parts of the body, with convulsive movements of the face and upper limbs, jaws firmly closed, eyes rolled up, and pupils dilated and insensible to light. Common salt, was given freely, and afterwards emollient drinks. When sensibility began to return, the patient suffered from violent epigastric pains. He ultimately recovered without serious consequences. The effects upon the nervous system may have been sympathetic with those upon the stomach, which was, no doubt, intensely irritated. (See Am. Journ. of Med. Sci., N. S., xxvi. 239.) Orfila found that from twenty to thirty-six grains, introduced into the stomach of a dog, caused death with the phenomena of corrosive poisoning, and that ulceration in the alimentary canal was produced. The fact probably is, that large doses may be borne without immediately fatal consequences, partly from the rapid chemical change which the salt undergoes, and partly from its superficial caustic operation, by which an extremely thin layer of epithelium is disorganized, and, forming with it an impermeable film, protects the deeper and more vital parts of the membrane from destruction. But, admitting this, it must be allowed that a frequent repetition of these large doses might gradually corrode into the deeper structure, and death ensue at last from the slow and continued operation of the poison. Esquirol mentions a case, in which nitrate of silver was given freely during a period of eighteen months, at the end of which time the patient died, and the stomach was found destitute of the mucous coat over one-half of the inner surface, with several points of corrosion to the peritoneal coat.

Nitrate of silver, therefore, must be ranked among the corrosive poisons. In acute cases of its poisonous operation, the treatment should consist in the use of common salt, or, if this is wanting, of the white of eggs, and a thorough washing out of the stomach by diluent drinks, or other means if necessary. Opiates should afterwards be administered, in the form of enema, to quiet irritation; and the resulting inflammation must be treated on general principles; care being taken not to exhaust the patient by too copious depletion.

Hitherto I have considered only the local operation of the medicine on the alimentary canal. It acts also on the system at large, through the circulation. Of its absorption, or rather of the absorption of the metal in one form or another, there can be no reasonable doubt. Leaving out of view the statements made as to the discovery of metallic silver in the pancreas after death, we have Orfila's declaration that he has found it in the liver of animals to which the nitrate has been administered for some time (see Lond. Med. Times and Gaz., March, 1852, p. 279); and, in addition to this, the indisputable evidence of the discoloration of skin produced by it. In any dose in which it is ordinarily given, its physiological influence upon the general functions is not striking. There may possibly be some increase in the frequency or force of the pulse, and the general temperature; but these results are much less observable than from the more powerful of the vegetable tonics. Salivation is said to have been sometimes produced, and writers speak of an eruption upon the skin; but I am not certain that I have witnessed either. One effect, however, is very striking. When the medicine is given in large doses, and long persevered with, a darkening of the skin is sometimes produced, which gradually deepens, till the whole surface of the body assumes a bluish-slate colour, extremely disagreeable to the eye, as totally differing from any natural tint. The discoloration is deepest in the parts exposed to light; but is said also to have been observed in the interior tissues, and in one instance to have pervaded the whole body; though this last statement may be considered as wanting confirmation. It is asserted that, in many instances, it does not occur until months after the medicine has been discontinued, and its favourable effects experienced. (Sigmund, London Lancet, March 31, 1838.) In connection with this fact, may be mentioned the detection by Orfila of silver in the liver of animals, to which the nitrate had been given, six months after its administration, showing that it is apt to become fixed in the tissues. In seven months, however, it had disappeared, and he could find no evidence of its existence in any part of the body. (Lond Med. Times and Gaz , March. 1852, p. 279.) So far as any inference can be drawn from this fact, it is that a patient cannot be considered free from the danger of discoloration in less than seven months after the omission of the medicine. The discoloration is generally permanent; but is said to have in some instances diminished with time, or under the operation of certain remedial measures. The seat of it is the true skin; for it remains unaffected when the cuticle is removed by a blister. The obvious explanation of the phenomenon is, that the preparation of silver circulating with the blood, when it enters the tissue of the true skin, and is thus exposed more or less to exterior influences, as of light and the air, undergoes decomposition, with the deposition of the partially reduced metal, probably in the state of a suboxide. This being insoluble in the blood, or in any liquid which the parts can generate, remains unabsorbed in the tissue. Happily, before the skin begins to exhibit this hue, a similar dark discoloration usually appears upon the gums and tongue and in the fauces, and serves, if observed, to put the practitioner upon his guard. It is said that the discoloration has disappeared under a steady course of cream of tartar. Dr. Pereira states, in his work on Materia Medica (3d ed.. page 886), that he has been informed of a case in which washes of dilute nitric acid diminished it. Iodide of potassium has been proposed as a remedy; but has proved ineffectual.

From what has been already said of the facility with which nitrate of silver is either decomposed, or otherwise rendered insoluble by chemical reaction, it is highly improbable that it ever enters the circulation without change. There are, however, two known states of combination, in which the metal may possibly be absorbed when the nitrate is administered. One of these is in the compound of nitrate of silver with albumen, which is soluble in an excess of albumen, and consequently in the blood, and the other in the double compound of chloride of silver and sodium, which is said to be soluble in water. (See pages 390-1.) As the albuminate undergoes the darkening process out of the body on exposure, it is highly probable that it undergoes the same change in the tissue of the skin, with a less degree of the same exposure.