The probabilities which have been established, that it is not unnatural to the blood, and is not always excreted from it, are in favour _à priori_ of its being Restorative. Catalytics are generally unnatural to the blood, and are excreted because they cannot remain in it. Besides, a Catalytic has generally some peculiar action on the blood in health; but a Restorative, in moderate doses, none,--only evidencing its operation when supplying a previous want. In this also Quina and its congeners agree with Restoratives and differ from Catalytic medicines. Arsenic is of use in Ague; and Arsenic is decidedly a Catalytic in all these particulars. In other respects, too, it differs widely from Quina. The latter is employed in debility, which depends evidently on some want in the system, and not on any morbid agent. Arsenic, on the contrary, is of use in Lepra, which, like other skin diseases, must be caused by some morbid agency, though we know not what precisely. But some diseases may be cured in two ways; either by the supply of something, or by the neutralization of something else. Perhaps Ague is one of these. And it is not repugnant to what we know of Ague to suppose that there is in it a want of some natural material, which would have, when present, the effect of checking the operation of the morbid agent. The fact of having had Ague once does not, as in the case of the Eruptive fevers, protect a man from the disorder thenceforward. So in this sense all persons may be said to be liable to Ague, and none protected from its assault. But it is not the case with Ague as it is with Syphilis and Small Pox, which diseases most persons inevitably catch who are exposed to the virus for the first time in their lives. For, of a number of persons exposed to the same malarious influence, only a part take the disorder; some escape. It is generally found that those are most likely to take it who have been previously debilitated by any cause; so that we must suppose that the rest have in their blood some material which serves to prevent the working in it of the Ague-poison, which apparently must enter it. It is not unlikely then that Ague may be cured by supplying the want of this material. Coupling with this consideration those facts which have been previously stated, we may reasonably conclude that Tonics are Restorative, and not Catalytic in their action; that they supply, or cause to be supplied, a material wanting in the blood. How this material is enabled to resist the morbid influence of the miasm--whether it is by an antiseptic property, such as has been attributed to Quina--I cannot determine. Having now done my best to establish the Proposition, as applicable to Tonic medicines, I may venture to bring forward a speculation concerning their action, which I would not wish to rate higher than it is worth, and still less endeavour to demonstrate as a fact.