When a disease depends on a want of some material in the system, then it admits of being cured by a restorative, which, in the theory of its action, is the simplest of all known medicines. And when a morbid process results in a diminution of the amount in the blood of some necessary constituent, then also may a Restorative be of use in alleviating the consequences of such a disorder; or may even effect a cure, when the morbid process has ceased, and left only its results behind it. But when the _materies morbi_ is still working in the blood, a radical cure can only be effected by a Catalytic medicine, which shall be capable of meeting it there, and of counteracting its noxious influence. Some Restoratives are also included among Catalytics, from a regard to a different phase of their action. The most important of these are Alkalies, which are of use in general inflammation, and also in Scrofulous and Scorbutic diseases, acting in a way that is at present but obscurely explained. When the _modus operandi_ of a medicine is obscure, but when it is apparent that it has the power of counteracting a disease, I have preferred to place it among Catalytics, rather than to include it with Restorative medicines. But there are not many cases in which we are thus left in doubt. The diseases curable by Catalytics are all evidently produced by an active morbid agency. Those which are treated by Restoratives are just as evidently remediable by the artificial supply or substitution of some one or more of the elements of the blood. CATALYTICS. These constitute the second division of Hæmatic medicines. The above name is applied to them on the assumption that their operation in the blood results in the destruction or counteraction of certain morbid agencies. ([Greek: kataluô].) The differences in action between this and the preceding division of blood-medicines have been stated at length in the remarks on Hæmatics in general. On recurring to Prop. VII., it will be seen that it is there stated-- "That others, (medicines of the first class,) called Catalytics, act so as to counteract a morbid material or process, and must pass out of the body." The action of the remedies in this division, which are the surest and most potent of all those that are employed in the treatment of disease, is involved in a greater degree of doubt and obscurity than that of any other class. Though there may be in some cases a certain dim explanation which we may catch hold of, and strive to fix or to render definite, yet in many instances there is not even so much as this. How can we, for example, by any of the common terms which are made use of to designate the actions of Mercury, of Arsenic, and of Iodine, express the peculiarity of their influence over Syphilis, Lepra, and Scrofula, respectively? Does it not seem better to confess our ignorance, and to say that all we know for certain is that these remedies have the power of antagonizing certain diseases?