Under the term resin a number of substances possessing certain properties in common are grouped together, but it is impossible to define the group as sharply as those of the fats, waxes, etc. The most important of these properties is insolubility in water, but ready solubility in alcohol, the alcoholic solution leaving on evaporation a film of transparent varnish. Resins are hard, amorphous, brittle solids (occasionally semi-solids), which soften when heated, and finally melt to a clear, adhesive fluid; they contain a large percentage of carbon, but only little oxygen, and burn with a smoky flame; they are free from nitrogen. Many are soluble in ether or in chloroform, but only a few dissolve in petroleum spirit.

Although most resins, and indeed all resins of medicinal or technical importance, are natural products, it is possible to produce resins artificially. Various volatile oils, especially those containing terpenes, yield resins by oxidation, and resinous bodies are also formed by the action of potassium hydroxide on aldehydes, etc.

Natural resins are commonly produced by special cells and discharged, together with other products, into a cavity formed by the separation of the cells from one another; in this way the oleo-resins of the Coniferoe, the gum-resins of the Umbelliferoe, etc, are produced. The tissue intervening between oleo-resin ducts frequently breaks down, so that branching cavities are formed; in this case the cell-walls appear to be converted into resin or oleo-resin, but the quantity thus produced is seldom large.

Guaiacum resin, on the other hand, is secreted in the vessels and cells of the heartwood, completely filling them and thus obstructing the passage of water. The resin of Indian hemp is secreted by external, stalked glands, whilst shellac is probably produced by the lac insect.

In addition to the secretion ducts normally present in the plant, others may be formed as the result of injury, and this may also take place even if the plant produces normally no such ducts. The number of the ducts thus formed may be very large and produce large quantities of oleo-resin which, discharged over the wound, forms a temporary protection for it. This flow of oleo-resin, which is termed ' secondary flow ' to distinguish from the ' primary flow' from ducts normally present, is the source of most of the oleo-resins of technical importance.

None of the resins are simple bodies; all are mixtures, and most of them are mixtures of complex nature.

The following are the chief classes of substances that have been isolated from the resins:

1. Resinotannols: aromatic, amorphous, brownish resin-alcohols allied to the tannins; they occur partly free, but more generally combined with aromatic acids or with umbelliferone in the form of esters (tannol-resins).

2. Resinols: crystalline, colourless resin-alcohols, also occurring partly free, partly in the form of esters (resinol-resins).

3. Resin-acids: these are partly crystalline, and mostly occur free; they combine with alkalies to form soaps, and with metals to form esters that often crystallise readily.

4. Resenes: these are indifferent bodies, and neither esters nor acids.

5. Glucoresins: these yield sugar by hydrolysis.

6. Various acids, more particularly belonging to the aromatic series, such as benzoic, cinnamic, paracumaric, salicylic, etc, which occur partly free and partly combined with the resin-alcohols.

The resins dealt with in this volume may be conveniently grouped according to their constituents, thus colophony, Burgundy pitch, sandarac, and amber are all of Coniferous origin, and present many points of analogy. Guaiacum contains chiefly resin-acids, and in mastich the principal constituent is a resene. Benzoin and dragon's blood contain resin-alcohols (resinols) together with aromatic acids. In shellac fatty resins are present while araroba, consisting almost entirely of crystalline substances, is not, strictly speaking, a resin, although it may conveniently be appended.