There is no principle in physiology better established than that which considers vitality as a power engaged in continual conflict with the physical, chemical, and mechanical laws, to which every species of inanimate matter is invariably subject. Every phenomenon of the living body might be advanced in illustration and support of this general position. The animal machine is constantly surrounded and assailed by agents, whose elective attractions for the principles of which it consists, are so numerous and energetic, that its decomposition would inevitably and speedily result, were not the adhesion of its molecules maintained by the conservative influence of a superior power. The compositions and decompositions which manifest themselves in the elaborate operations of chylification, sanguification, and secretion, are carried on by agencies totally distinct from those which govern the combinations of inert matter, and must be investigated upon principles essentially different. How then, it may be asked, can a medicinal substance be brought to act _chemically_ upon the _living_ body? Notwithstanding the general proposition, that the animal processes to which we have alluded, are governed by laws peculiar to life, yet it must be admitted that such processes are occasionally influenced,[177] modified, and controlled by powers strictly chemical in their operation; although in some cases it will be seen that such effects afford only apparent exceptions to the general law: for several of the remedies whose operations have been regarded as purely chemical, exert their influence on parts which cannot be strictly considered under the control of the living principle; of which _Antacids_, and certain _Antidotes_ and _Antiseptics_, to be hereafter explained, may be considered as examples: in like manner will cataplasms of acetic acid hasten the exfoliation of carious bone; a practice which has lately been employed with much success in the Infirmary at Gloucester. Upon the same principle alkaline applications may be made to dissolve coagulated blood; suppose, for the sake of illustration, that the bladder should become filled with coagulum through hemorrhage from the prostate gland, and that the most serious consequences were to be apprehended from the distention; in such an emergency, a dilute and tepid solution of potass, if injected through the catheter, might prove eminently serviceable; although such a practice would require the utmost skill for its safe direction, since the removal of the plug, thus afforded by Nature to the bleeding vessel, might be followed by an immediate return of active hemorrhage. On the contrary, other agents destroy the vitality of the organ before they can produce any change in the matter of which it is composed, as the action of _Escharotics_ will clearly demonstrate. REFRIGERANTS.