The principal fixed oils which are used in medicine are Castor, Olive, Almond, Croton, and Cod oils. It is certainly to be regarded as a very beautiful arrangement in the animal economy, that those substances which are not acted upon by an acid fluid should be subsequently subjected to the action of a free alkali, so that by the successive action of these solvents, together with the peculiar process of stomach digestion, the great majority of substances taken into the system are dissolved and rendered fit for absorption. There seem to be other vegetable substances which are rendered soluble by means of alkali in the way already described. Resins form an important class of remedial agents, in which are comprised many Diuretics, Diaphoretics, and Purgatives. They consist chemically of peculiar acids, which, though themselves insoluble in water, combine with alkalies to form salts which are soluble. They are certainly in most cases absorbed. They have been found in the blood, and detected when passing out in the urine. From this last they may be precipitated by an acid, indicating that they are held in solution by an alkali. In large doses they may not be absorbed, but by irritating the surface of the intestinal canal, may act externally as Cathartics, and be expelled by the peristaltic action which they excite. But in small doses they enter the circulation in solution, and affect remote organs. The only way in which they can be dissolved is by means of the alkali of the two intestinal fluids. Among resinous medicines I may mention Catechu, Kino, Benzoin, Storax, Peru and Tolu, Copaiba, Guaiacum, the fetid resins, etc. Many purgative drugs, as Jalap, Scammony, and Gamboge, owe their efficacy to resin. There are moreover certain neutral acrid principles, similar in their nature to resins, which are soluble only in alkalies, and thus come under this head. Such are Cantharidin, Piperin, Pyrethrin, Colocynthin, Elaterin, and Capsicin, obtained from Cantharides, Pepper, Pyrethrum, Colocynth, Elaterium, and Capsicum. But it should be observed that some of these are soluble in Acetic acid; and if, as some have supposed, this acid exist in the gastric juice, then they might be dissolved in the stomach. We may add here some medicines which were enumerated also in a former division. Creosote, not very soluble in water, is easily dissolved in a free alkali. This substance also, like the last, is soluble in Acetic acid. Some volatile oils, especially Turpentine, display a marked tendency to oxidize into resins; and being themselves very sparingly soluble in water, may perhaps be dissolved after having undergone this change. Thus Turpentine changes into common Resin, which consists of two isomeric acids, Pinic and Sylvic. Turpentine = C_{20}H_{16} Pinic Acid = C_{20}H_{15}O_{2} and, C_{20}H_{16} + 3O = C_{20}H_{15}O_{2} + HO.