particles of a stimulant medicine may be of such a nature as to irritate, or to refuse to coincide with, the ultimate molecules of the sensitive nerve with which they come in contact; and those of a sedative may, on the other hand, be so shaped and arranged as to dove-tail with those particles, and by extinguishing, as it were, their salient points, to cloak their vital sensibility. This is obviously a mere conjecture, and the only value which can attach to it is, that it appears in some sort to explain a thing which without it is inexplicable. Some modern writers have attempted to clear up the actions of certain medicines by their supposed influence on the physical process of endosmosis, as carried on through the coats of the stomach and intestines. Poissenille and Matteucci have attempted to prove that the action of saline liquids in causing purging, and that of a solution of morphia in preventing the same, may be explained by the _endosmotic_ properties of these liquids, as ascertained by experiment. It does not seem to me that these ideas can be successfully maintained. (_Vide_ Prop. II.) 2. Several attempts have been made to explain the general action of medicines on chemical principles. Perhaps the strange doctrine taught by Galen, which prevailed for so many centuries afterwards, should be mentioned under this head as the first approach to a chemical theory. He considered all medicines to be _hot_, _cold_, _moist_, or _dry_. There were four degrees of each of these properties. In the _Pharmacopæia Londinensis_ of 1702, translated by Dr. Salmon, it is stated of every herb that it possesses in a certain degree one or more of these qualities. It is amusing to find Dr. Salmon in great doubt as to whether Opium were _hot_ or _cold_, as the Ancients said one thing, and the Moderns another. Galen supposed that diseases depended on similar qualities, and were to be counteracted by medicines; that, for example, we were to meet a _hot_ disease by a _cold_ remedy.