The action of Catalytics is more obscure than that of Restoratives, but there is generally a broad line between them. The most difficult case is that of the medicines used in ague and other periodic diseases; of these, for certain reasons that I shall state, I have grouped Quina and tonics among Restoratives, placing Arsenic among Catalytic medicines. It seems to me that Cinchona bears the same relation to Arsenic in the treatment of ague, as Alkalies to Colchicum in gout; the former, in each case, supplying a needful material, the latter neutralizing a morbid process. Thus, when air has been rendered impure by breathing, we may render it again respirable by adding fresh oxygen; but still more certainly, by neutralizing the carbonic acid. ORD. IV. TONICS. Under this term I intend to include only the bitter principles of vegetables. The chief medicines of this order are as follow: _Alkaloids and neutral principles._--Quina, Cinchonia, Bebeerine, Narcotine, Salicine, etc. _Bitter drugs._--Cinchona, Quassia, Cusparia, Gentian, Calumba, Chiretta, Centaurium, Menyanthes, Rhubarb, Hops, Elm and Willow barks, Tansy, Wormwood, Chamomile, Cascarilla. The power by virtue of which these medicines are enabled to act as Tonics, is due, in each case, to a vegetable alkaloid or neutral principle, having a bitter taste. All of those mentioned above, with the exception of Salicine, contain Nitrogen. The uses of Tonics are of a twofold nature. Firstly, they are of use in simple debility and in atonic dyspepsia. Here they give an appetite, increase the muscular strength and powers of digestion, and improve the general health. In the second place, they have all, more or less, a curative power in Ague and other periodic disorders, caused by Marsh miasm. Many explanations have been given of this curious and double action of vegetable bitters; and some erroneous theories have been assumed, and false analogies attempted, in the classification of these remedies. The subject is certainly a difficult one, and there are, at starting, several questions which require a distinct consideration and reply. Upon the answers given to them it must depend whether they should rightly be grouped with blood-medicines or with nerve-medicines; and, if the former, with Restoratives or with Catalytics. Do these medicines act primarily in the blood, or on the nerves? Is their action of a permanent character? Are there any substances in the blood which resemble them? May they remain in the blood, or are they always excreted? If acting in the blood, are they wont to effect a cure by supplying to it a material wanting, or by counteracting in it a morbid process? These questions require each a satisfactory reply, before it can be proved that I have done rightly in classing Tonics among Restorative medicines; for they coincide with the minor propositions in which I have defined the action of such remedial agents.