This might, perhaps, be included in some one or more of the processes already referred to. Substances may be employed therapeutically, in reference to their chemical influence, for three purposes; first, for the destruction of the tissues, as in the formation of issues, the removal of morbid ulcerated surfaces, etc.; secondly, for the neutralization, or other chemical change of substances contained within the body, but not form-ing an essential part of it, as when excess of acid in the primae viae, the blood, or the urine, is obviated by alkalies, or an insoluble metallic combination in the tissues is rendered soluble by the chemical agent administered; and, thirdly, through their reaction with the constituents of the blood or of the tissues, to produce changes in them favourable to the removal of disease. But their influence in all these methods of action may be resolved either into contra-causation, elimination, or alteration; except in the formation of issues, in which instance the chemical action is not in itself curative, but simply operates by setting on foot certain physiological processes which constitute the real remedy in the case. In relation to the process of alteration, it is highly probable that, in many instances, it is purely the result of chemical reactions set on foot by the remedy in the interior of the system; but we have little positive knowledge upon the subject, and theoretical speculations can lead to little practical good, except in so far as they may serve as a guide to inquiry and experiment. They should not be allowed to serve as the basis of curative methods, until the chemical reactions have been experimentally traced out, and demonstrated beyond reasonable doubt.