_Sulphate of Magnesia_ does not readily produce any diuresis, because it operates upon the bowels, but the experiments of Vitet and Bracy Clarke have shewn, that if this saline compound be administered to the horse whose bowels are not easily affected by purgatives, it acts powerfully upon the kidneys;[159] and I will take occasion in this place to observe that, on account of the inirritability of the bowels of the horse, diuretic medicines are more certain in their operation, than in the human subject; a fact which, in itself, shews the importance of attending to the state of the bowels, during a course of those diuretics which require to be absorbed before they can produce their specific effects. Equally necessary is it to attend to the state of the vessels of the skin, for if during the administration of a Diuretic, these vessels be excited by external warmth, its action may be diverted from the urinary organs to the exhalants on the surface, and occasion diaphoresis; but if the surface of the body be kept cool, this diversion will not occur: so greatly indeed does cooling the surface determine to the kidneys, that the usual diaphoretic medicines may, by an attention to this circumstance, be converted into powerful diuretics. C. II. MEDICINES WHICH ACT _Primarily_ ON THE ABSORBENTS, AND _Secondarily_ ON THE KIDNEYS. It has been shewn, in the former division, that by increasing the action of the kidneys, we diminish the quantity of water in the blood, and consequently occasion an extraordinary action of the absorbents to supply the deficiency, whence dropsical accumulations disappear; it remains to be stated that an operation, which may be considered the converse of the one just described, is not unfrequently established: the absorbent vessels, in this case, are first roused to extraordinary action, and the blood therefore becomes surcharged with serous matter, in consequence of which the kidneys are stimulated, and it is eliminated through the urinary passages: so that in the former case the absorbent may be said to be called into action by the kidneys, while in the latter, the kidneys are obviously subservient to the increased energy of the absorbent system. The preparations of Mercury are perhaps the only medicinal bodies which we can strictly consider as specific stimulants to the absorbent system; and of their power in directly acting upon these organs there are such ample proofs, that it is unnecessary to adduce any additional evidence upon the subject. In instances of increased absorption from the agency of other medicines, the effect must be considered as rather arising from their _secondary_ than primary operations; the most important of which will constitute objects of inquiry in the succeeding divisions of the subject. C. III. MEDICINES WHICH ACT _Primarily_ ON THE STOMACH OR SYSTEM, AND _Secondarily_ ON THE URINARY ORGANS.