_m. p._ 5.--That these medicines are all unnatural to the blood, and must at length pass out of the system. It might seem at first as if the first, fourth and fifth of these minor propositions would alone suffice to include the original affirmation. But a medicine may act in the blood, and may counteract a disorder; and yet it may counteract the disorder in some other way than by acting in the blood. It is necessary further to prove the disease to be a blood disease. And the evidence of counteraction will be rendered stronger if we can prove that the medicine employed is itself capable of setting up in the blood some peculiar process--of causing in it a change of some special kind. For, were it not to do so, we could hardly understand how it could meet the requirements of one case more than of another, or how it could at all arrest an active process in that fluid over which it exerted itself no particular influence. And this thing, which is laid down in the second minor proposition, can, I think, be proved in the great majority of instances. The following is the arrangement of the subject which I have thought it best to adopt. We will, in the first place, consider some of the statements ventured by authors respecting this kind of medicines: as a comparison of these should aid us in the discovery of the truth. In the second place, it will be convenient to enumerate the medicines contained in the orders named above, and to proceed to prove the minor propositions of Catalytics taken altogether. A considerable amount of difficulty attaches to this consideration; and, as the argument which applies to one order will apply to all, much unnecessary repetition will be avoided by this plan. In the third place I must conclude by making some remarks on each of the orders in succession. To proceed now to the first of these topics. In referring to the opinions that have been advanced on the action of this group of medicines, our task is rendered comparatively easy by the fact that a great number of writers on Materia Medica have failed to recognise them collectively. It may seem strange to one who has reflected on the subject, but it is nevertheless true, that the majority of authors have taken no notice whatever of medicines acting in the blood, with the exception of Acids and Alkalies, which are grouped by some as chemical remedies. All other medicines have been generally arranged under the two great and ill-defined heads of Nerve-medicines and Eliminatives. I cannot find it possible, by either of these terms, to give any idea of the real action of the many important agents which I have included in this division.