In some affections, a deficient diaphoresis may be associated with increased vascular action, and in others, with a slow languid circulation. Diaphoretics may be considered as operating, either by directly stimulating the cutaneous capillaries;—by increasing the general action of the vascular system;—by relaxing the morbidly constricted mouths of the perspiratory vessels;—or, lastly, by producing at once both the latter of these effects. In conformity with the plan adopted on other occasions, I shall proceed to investigate the powers of this class of medicines, according to their supposed modes of operation. DIAPHORETICS Occasion their effects— I. BY STIMULATING THE CUTANEOUS CAPILLARIES. _A._ By external application. _The Stimulus of Heat_, _Frictions_, _&c._ _B._ By Medicines which enter the circulation and stimulate the cutaneous vessels _by contact_. _Mercurials_—_Sulphur_. _C._ By Medicines which act on the surface _sympathetically_, through the medium of the Stomach. _Cold Drinks_, _&c._ II. BY INCREASING THE GENERAL ACTION OF THE VASCULAR SYSTEM. _Violent Exercise_—_Ammonia_—_Guaiacum_—_Alcohol_—_Warm Bath_. III. BY RELAXING THE MORBIDLY CONSTRICTED MOUTHS OF THE PERSPIRATORY VESSELS. _Antimonials_—_Cold Affusion_—_Venesection_—_Saline Diaphoretics_. The action of the cutaneous vessels may be augmented by heat, without necessarily increasing, at the same time, that of the heart and arteries; hence it is that heat is, of itself, often sufficient to produce sweating, while it generally accelerates the operation of a sudorific medicine. To this general proposition, however, there are some very important exceptions; and, indeed, in certain conditions of the cutaneous surface, the stimulus of heat will be even found to impede, rather than to promote, diaphoresis; thus in the hot stage of a continued fever, there would seem to exist a peculiar constriction of the perspiratory vessels, accompanied with extreme heat and dryness. In such a state, remedies of the third class must be applied, or conjoined with those of the former. The warm bath may be said to partake of _all_ the qualities upon which our classification is founded; it will stimulate the cutaneous capillaries,—increase vascular action, generally, and, by its emollient powers, relax the morbidly constricted mouths of the perspiratory vessels. During the ardent heat of fever, the external application of cold is the most efficient sudorific, as the valuable reports of Dr. Currie have very satisfactorily established.