MODE OF ACTION OF MEDICINES.

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silver, and, perhaps, powdered tin ; the first and the last are used asanthelmintics—the second, to overcome intus-susception, or intestinalinvagination.

b. Chemical .—If substances, having powerful affinities for organic mat-ter, be applied to the living tissues, they first destroy the life of the part,and then enter into combination with one or more constituents of thetissues: such substances are termed caustics. But the destruction oflife in one part is attended with alterations in the vital actions, and theproduction of inflammation in surrounding parts; so that the chemicalaction of caustics is attended by both chemical and vital effects, and thewhole of their operation may be denominated a chemico-vital process.

If the energy of the affinity of caustics for organic matter be diminished,as by diluting them, the vital powers are sometimes enabled to resist theproduction of any immediate chemical change, and the life of the part isconsequently preserved. The caustic, then, operates at first as a mereirritant, and causes alterations of vital action only. In this case theactive force is still supposed to be affinity; that is, the particles of thecaustic are presumed to have a tendency to unite with those of the orga-nised tissues; but the union being resisted by the vital powers, a newaction is set up which constitutes the changes or effects before referredto. The long-continued application, however, of these weak chemicalagents, will gradually effect slight changes in the composition of thetissues without producing the death of the altered parts. These organicalterations of a living part are of course attended by the production ofmorbid actions.

Chemical changes are sometimes produced in the secretions of distantparts by the internal use of certain agents. Thus the qualities of theurine are modified by the administration of acids or alkalis. Do thesemodifications or changes depend on the chemical influence of the sub-stances swallowed ? or on some other kind of influence which theseagents exert, either directly or indirectly, over the secreting organs ?Neither explanation is without difficulties; but I conceive the first topresent the fewest. It cannot be denied that when either alkalis or acidsare swallowed, they pass out of the system, in part at least, by thekidneys; and that in the urine they possess their usual chemical proper-ties, modified by the presence of any substances with which they mayhave united. Moreover, the qualities which they impress on the urineare similar to those which they produce when added to this secretion afterits evacuation from the bladder. Thus, by the internal use of alkalis, ithas been found that the natural acidity of the urine may be destroyed, andan alkaline quality substituted for it: the same condition of mine is pro-duced by the addition of alkalis to this fluid out of the body. Again, theinternal use of soda or magnesia may give rise to the appearance of whitesand (phosphates) in the urine : now the same kind of deposit may be pro-duced in healthy mine by the addition of a few drops of an alkaline solutionto it. Furthermore, by the administration of acids (sulphuric or hydro-chloric), phosphatic deposits are diminished or entirely prevented, whilethe employment of alkalis promotes them. Now this influence of acids isprobably, in part at least, chemical, since we find that a few drops addedto urine which contains these deposits, dissolves them. In other words,as tie modifications which acids and alkalis produce in the condition ofthe rrine are precisely those which we might expect from the known