Of the list of Catalytics, the only medicines that occur as constituents of healthy blood, are Alkalies, Salts of the alkalies and earths, Chlorine, and Sulphur. Of these, the Alkalies, and possibly also the others, are not unnatural to the blood when administered in small quantities, and may remain in the system and act as Restoratives, when there is a deficiency in the blood of similar materials. But even these substances, when given in large quantity, as is the case when they are used for Catalytic purposes, are unnatural to the blood, and must be excreted from it. With respect to the other Catalytics, they cannot any of them remain naturally in the blood, under any circumstances. Their very presence for awhile constitutes an artificial disease, and is only to be tolerated or recommended because it may serve to counteract a morbid action of a more serious and uncontrollable character. The kidneys generally constitute the channel by which these Catalytics are removed from the blood. Most of them, in passing out, act as diuretics. To this we must, perhaps, except the salts of lead, zinc, and copper. Doubtless some are excreted partially or entirely by the mucous membrane of the bowels, but this we cannot so readily appreciate. The circumstance that the astringents just mentioned are efficacious in diarrh[oe]a seems to point to their access to the intestinal surface from the blood. It has already been shown that these Catalytics are all soluble in some way in the intestinal canal, and that they are absorbed. (_Vide Prop. I.; Prop. II._) All of them that can be detected by chemical means, have actually been discovered in the blood. But the system will not, if it can be avoided, suffer them to remain there long. The glands are charged with the office of purging the blood of all morbid materials; and thus these substances pass out in the secretions; most particularly, as I have just said, in the secretion of urine. Every one of the medicines of this division, enumerated above,--excepting Colchicum and Pitch, which are difficult to recognise chemically,--have been detected in the urine by M. Wöhler, M. Orfila, and others. The Alkalies and Acids are combined, so as to form salts; the vegetable salts are decomposed; Sulphur has changed into Sulphuric acid; and the metallic bases are found to be variously combined; but all the fixed inorganic materials remain essentially the same, however altered in arrangement. Many of these medicines have been likewise detected in the secretions of other glands. This subject will be further discussed in the consideration of Prop. X., and does not immediately concern us now. Thus the minor propositions may be considered as proved; and all that I have ventured to affirm of this group of medicines is, that they counteract morbid agencies by an operation in the blood.