Now though I have a probable hypothesis to advance as to the action of some of these, I would not have this considered as more than hypothetical. I would not speak positively of the action of any one of them, any more than to say that each of them tends to neutralize one or more particular morbid poisons. Some have tried to give a general explanation of their action, and have talked of it as if it were easily understood by their known properties. I am not of their opinion; and when I speak of Catalytics, I shall give my reasons for disagreeing with them. Now each Catalytic has peculiarities and affinities that distinguish it from all others. I have not thought that I could arrange them more truthfully than by subdividing them according to diseases which they tend to counteract. How inadequately is the action of Mercury and of Iodine expressed by calling them special stimulants, alteratives, or absorbents! Is it not better and more correct to say at once that Mercury is useful in checking inflammation in general, and in counteracting the poison of Syphilis in particular? and that Iodine is effective in secondary forms of the latter disorder, as well as in Scrofula? These medicines, then, are specifics, in so far as they are particularly useful in certain disorders, and in those excel other remedies; but they are not, in the vulgar acceptation of that term, the only medicines which can be employed in such a disease, nor is their use to be restricted to it alone. I have already said that a Catalytic tends of itself to work out a peculiar process in the blood. For this reason their administration in health generally does harm. They have nothing in the blood corresponding to them; or if so, they are not introduced to supply its want. Thus they must eventually pass out of the body. Before doing so, some may act on the nerves. While so passing out, they may, as I have said, act either as Astringents or Eliminatives on the glands. Under these heads their secondary action will be subsequently considered. Being thus foreign to the blood, Catalytics do not remain there to supply a want; but just long enough to counteract a morbid action, and are then excreted.