It may be observed that this idea of action by contact cannot by any means be supposed to favour the very unreasonable theory of the efficacy of infinitesimal doses of drugs. For such medicines must of necessity be present in some amount, or they cannot act at all. Their operation cannot be like that of the putrefying yeast, or of the poisons of contagious fevers, each of which can reproduce itself out of the elements of the changing or fermenting body. For Antimony, Mercury, and Iodine could never make themselves out of blood, which does not contain them. Like the Emulsine in the production of Hydrocyanic acid out of the material of the bitter almond, they must be present in certain quantity, or they are quite inoperative. Even a tenth of a grain would generally be powerless, not to mention such irrational quantities as the thousandth, millionth, or even decillionth of a grain. (_Vide_ page 54.) Experience--better even than theories and mathematics--is entirely opposed to such chimerical fancies. Having thus entered into a speculation concerning the probable action in the blood of Catalytic remedies, I must once more remark that this idea forms no part of the Proposition in which I have defined their mode of operation. In the present state of our information on the subject, we cannot certainly say more than that these remedies, by some blood-action, are able to antagonize and to annihilate certain disorders in the blood. The resolution of a disease thus effected may, in some few instances, be partly explained by certain chemical considerations, as will be particularly shown in the case of Antiarthritics. It remains for me now to add some brief remarks on the individual _modus operandi_ of the substances included in each of the eight orders of Catalytic medicines. Some of these having been partly discussed above, in arguing the Proposition, it will not be found necessary to travel over again that portion of the ground which has already been thus traversed. ORD. I. ANTIPHLOGISTICS. The inflammatory process--the chief seat of which, however it may first originate, is in the circulating blood--exerts a powerful influence over the nervous system, and may, by this influence, cause death. When fever is produced to any extent, there must be danger. We are enabled in two ways to control or to mitigate this state of things. We may produce an action on the nervous system, or we may direct our attention to the process in the blood.