Gums are exudations from the steins of plants. They consist of two rather complex carbohydrates, arabin, C12H12O11, and bassorin, C12H10O10, which play the part of acid radicals, and exist in gums as salts of magnesium and potassium. Arabin is soluble in water; bassorin is not soluble, but swells into a gelatinoid mass. Pectin, vegetable jelly, C32H40O28, 4H1O, occurs in a few medicinal plants, and, like the mucilage yielded by several others, is allied to gum. Gum-resins are natural or artificial exudations from plants, containing various proportions of gums and resins, or more frequently of gums, resins, and volatile oils.

Alkaloids are - active principles formed within plants, which resemble alkalies in turning red litmus-paper blue, and form salts with acids. As a rule, they are crystalline solids, rarely liquids; sparingly soluble in water, but readily in alcohol, the solution being intensely bitter.

Organic acids of great variety exist in plants, combined with the inorganic bases, such as potash and lime, with alkaloids, or possibly free.

Neutral substances are a very large and mixed group, including the carbohydrates, such as starch, sugars, gums, etc.; albuminous bodies, which occasionally act as ferments; a few bitter principles; and glucosides.

Glucosides are chiefly neutral bodies, capable of being decomposed in the presence of water into glucose and a second substance, different in each instance.

The remaining constituents of organic drugs do not call for special notice.

Dose. - The Pharmacopoeia suggests the limits within which the different substances and their preparations may be safely given to an adult. These must be carefully learned. The principles of dos will be presently discussed.

Preparations. - The list of preparations made from the drug, with the principal ingredients, strength, and doses of each, will conclude the account of its pharmacy. This subject demands special consideration here.

Most of the materiae medicae possess such characters that it is absolutely necessary to prepare them for administration. Thus, if we take, as examples, Sulphur, one of the elements; Potassii Iodidum, a crystalline salt; Chloroformum, a liquid compound of chlorine and formyl; Colocynthidis Pulpa, the dried pulp of a fruit; Jalapa, a tuber; and Cantharis, a dried beetle; it is manifest that few of these can be brought into useful contact with the body in their native form. Preparations must be made from them, and for several reasons we must have a variety of preparations. First, as we have just seen, substances are very various; secondly, a substance may contain several active principles, soluble in different media, which it may or may not be desirable to extract together or separately; thirdly, we constantly wish to obtain combinations of drugs, so as to increase, diminish, or otherwise modify the action of each, or to obtain combined action; fourthly, we must provide for variety of administration or application, externally or internally, to act on a part or to enter the blood by any of the methods of exhibition to be presently described; and we must be ready to meet the tastes and fancies of patients with respect to pills, powders, etc., as well as the necessities of circumstances.

The following is a list of the different kinds of preparations in the British Pharmacopoeia. A complete list of each will be found in the synoptical tables at the end of the volume.

Aceta, Vinegars, are extractive solutions in acetic acid (not vinegar).

Aquae, Waters, are very weak simple solutions of volatile oils in distilled water, obtained by distilling the vegetable products or the volatile oil. Aqua Camphorae is a solution without distillation. Aqua Chloroformi is the only aqua not made from an oil.

Cataplasmata, Poultices, are familiar external applications. They generally contain linseed meal as their basis.

Chartae, Papers, consist of cartridge paper coated with an active compound much like a plaster.

Confectiones, Confections, conserves, or electuaries, are soft pasty-looking preparations, in which drugs, generally dry, are"incorporated with syrup, sugar, or honey.

Decocta, Decoctions, are made by boiling vegetable substances in water from five to twenty minutes. All decoctions are simple, except that of aloes and one of the decoctions of sarsa.

Emplastra, Plasters, are external applications which adhere when applied to the body, and produce either a local or a general effect. The basis in all is a compound of fatty substances (resin, wax, lead, soap, etc.), and is intended to be spread on linen, leather, or other material.

Enemata, Enemas, injections, clysters, are liquid preparations for injection per rectum. The basis is generally mucilage of starch or water.

Essentiae, Essences, are solutions of volatile oils in four parts of rectified spirit, i.e. are ten times the strength of the ordinary spirits.

Extracta, Extracts, are preparations obtained by evaporating either the expressed juice of fresh plants or the soluble parts of dried drugs. They are, therefore, of several kinds:

1. Green Extracts

Green Extracts. The juice pressed from the bruised plant is heated to 130°, to coagulate the green colouring matter, which is strained oif and reserved. The fluid is next heated to 200°, to coagulate the albumen, which is separated by filtration and rejected. The filtrate is now evaporated at l40° to a syrup, the green colouring matter returned, and the whole evaporated down to the required consistence. Ex.: Extractum Aconiti.

2. Fresh extracts are prepared like green extracts, but there being no colouring matter, the juice is heated at once to 212° Fahr. to coagulate the albumen, filtered, and evaporated at 160°. Ex.: Extractum Taraxaci.

3. Aqueous extracts are prepared from drugs by the action of cold, hot, or boiling water on dry drugs, and subsequent evaporation to a proper consistence. Ex.: Extractum Calumbae, Extractum Gentianse.