Diuretics arc medicines which promote the secretion of urine. They may be supposed to operate in one of two methods; either by a sympathetic impression extended from the stomach or other surface of application, through the nervous centres, to the kidneys; or by entering the circulation, and stimulating the glands directly, through contact with their secretory structure. There arc other modes of increasing the secretion, to which reference will be made directly; but the agents, in these cases, have no peculiar or specific tendency to act on the kidneys; and the effect is ascribable to certain physiological laws, brought into operation by influences, which, under other circumstances, would equally operate on other functions. Each of the methods above mentioned requires a few words of explanation.

1. Operation by Sympathy. Before the fact of the general absorption of medicines had been established by the irresistible evidence of experiment, it was a common belief that the diuretics, as well as most other medicines, acted on parts distant from their seat of application, through the principle of sympathy; in other words, by communication through the nervous centres. A diuretic, admitted into the stomach, made a certain impression upon the mucous coat, which was transmitted to the sympathetic or spinal centres, whence an influence was sent to the kidneys, exciting them to an increased performance of their function. At present it would be difficult to point out a single diuretic medicine, of which it could be reasonably said that it even probably acts in this way; and the only influences, so far as I am aware, that are capable of such an operation through the nerves, are cold applied to any part of the body, and mental emotion acting upon the cerebral centres. Of these more will be said immediately.

2. Operation through Absorption. The method in which all the proper diuretic medicines act upon the kidneys is probably by direct contact. in relation to many substances which belong to the class, it has been determined by experiment that they are absorbed into the circulation, and escape from it through the kidneys. One great office of these glands appears to be, to depurate the blood of those foreign bodies which find their way into it, and which might otherwise do injury to the system. Hence, their susceptibilities have been made such that they feel the presence in the blood of these noxious agents, and are stimulated by them into excessive action in order that they may be thrown off. But, in separating the substances referred to from the circulating mass, the kidneys are compelled to eliminate with them sp large proportion of the liquid in which they are dissolved. Thus, most medicines of an acrid or stimulant character which enter the circulation, and many others which can scarcely be said to have this character, more or less increase the secretion of urine; but it is only to a few of them that the distinctive title of the class is attached; as but a few exercise the property to such an extent, and so uniformly, as to be capable of beneficial employment as mere diuretics. Allusion was made above to certain influences, not properly medicinal, which have the effect of increasing the action of the kidneys. The operation of diuretics cannot be fully understood, without attention to these influences. They may be arranged under the heads of cold, vas cular fulness, arterial stimulation, and mental emotion.