So far the conditions of sweating and of diuresis are nearly opposite. But this is not the case with the medicines which are used to cause them. Diluents, and salines soluble in water, form the first group of true Diaphoretics. Water promotes alike the function of the skin and of the kidneys; and it is only by a regulation of the circumstances mentioned above that it can be diverted from the latter towards the former. Diluent drinks are indispensable adjuncts to a Diaphoretic regimen. Salines also tend naturally to pass off in the urine, when in small doses; but when in large amount, by the bowels. A saline, being soluble in water, cannot pass out except into a fluid secretion; so that a saline diaphoretic should be given in a moderate dose, and to secure its action the skin should be kept covered and warm, and the patient in a recumbent posture. The medicine will then pass off into the liquid sweat, as it otherwise would have passed into the urine. Volatile Diaphoretics may increase the cutaneous transpiration, and pass off by the skin, without the production of sensible liquid sweating. For being soluble in air, and capable of being carried away by it, they therefore tend especially to the two aeriform secretions, _i.e._ those of the lungs and of the cutaneous surface. (_Vide_ pp. 278, 285.) Ammonia and its various salts are very useful as Diaphoretics. Volatile oils are less energetic, for they often pass off with such ease as not sensibly to increase the secretion of the skin. Some acrid matters, as Guaiacum, Mezereon, and Senega, appear to act specifically on the function of the skin. So also do certain Narcotics, among which Opium is conspicuous as the most certain Diaphoretic that we possess, though its other operations prevent it from being applicable in all cases. Antimony, Mercury, and Sulphur, are medicines which appear to a greater or less degree to increase all the secretions in the body. When given in the insoluble form, they are reduced by the system to a soluble state. When Tartar Emetic is given in small doses, its only apparent action is slightly to increase the perspiration. It is probable that it then passes out through the glands of the skin. When given in emetic doses, it may produce sweating indirectly, as mentioned above. Iodine, which also increases secretion generally, acts sometimes as a Diaphoretic. Many volatile oils have been detected by their odour in the perspiration, as those of Garlic, Onion, Asaf[oe]tida, Musk, and Copaiba. Mercury, Sulphur, and Iodine, have been detected chemically in it. It has happened, when a course of Mercury has followed the administration of Sulphur, that parts of the skin have turned black, from the formation of Sulphuret of Mercury. So it is proved of many of these Diaphoretics, and thus rendered highly probable of the rest, that they are true Eliminatives, being themselves excreted by the glands which they excite to action.