RESINS, a class of natural vegetable products composed of carbon, hydrogen, and oxygen. They are closely allied to the essential oils, all of which when exposed to the air absorb oxygen, and finally become converted into substances having the characters of resin; and in most cases they are obtained from the plants which yield them, mixed with and dissolved in a corresponding essential oil. Like the natural oils the natural resins are usually mixtures of two or more distinct resins, which admit of separation by their unequal solubility in different fluids.
The following are the general characters of this class of compounds: At ordinary temperatures they are solid, translucent, and for the most part colored, although some are colorless and transparent. Some are devoid of odor, while others give off an aro matic fragrance from the admixture of an essential oil. In their crude state they never crystallize, but are amorphous and brittle, breaking with a conchoidal fracture; when pure several • of them may, however, be obtained in the crystalline form. are readily melted by the action of heat, and are inflammable, burning with a white smoky flame. They are usually described as non-volatile, but it has been recently shown that common resin may be distilled in a current of superheated steam. They are insoluble in water, but dissolve in alcohol, ether, and the essential and fixed oils. They are insulators or non-conductors of electricity, and become negatively electric by friction. Many of them possess acid properties, tit which ease their alcoholic solutions redden litmus. These resins combine with the alkalies, and form frothy soap-like solutions in alkaline lyes. The resinous soaps thus formed differ from ordinary soap in not being precipitated by chloride of sodium.
The resins are divisible into the hard resins, the soft resins, and the hard resins are at ordinary temperatures solid and brittle; they are easily pulverized, and contain little or no essential oil. Under this head are included copal, the varieties of lac, mastic, and sandarach, and the resins of benzoin (commonly called gum-benzoin). jalap, guaiacum, etc.—The soft resins admit of being molded by the hand, and some of them are viscous and semi-fluid, in which case they are termed balsams. They consist essentially of solutions of hard resins in essential oils, or admixtures of the two. They become oxidized and hardened by exposure to the air into the first class of resins. Under this head are placed turpentine, storax, balsam of copaiva, and the balsams of Canada, Peru, and Tolu.
. The gum-resins are the milky juices of certain plants solidified by exposure to the air. They consist of a mixture of resins and essential oils with a considerable proportion of gum; and on this account, when rubbed up with water, they yield a turbid or milky fluid from the dissolved gum, retaining the resin and oil in suspension, and are only partly soluble in alcohol. Some of them, as ammoniacum, assafetida, euphorbiurn, galbanum, gamboge, myrrh, olibanum, etc., are valuable medicinal agents; while others,
as caoutchouc (or india-rubber) and gutta-percha, are of great value in the arts and in manufactures.
The resins are very widely diffused throughout the vegetable kingdom. But there are certain families of plants which are especially rich in them. They are generally obtained by making incisions into the wood of the trees which produce •hem; some times, however, they exude spontaneously, and in other cases require to be extracted from the wood by boiling alcohol. The crude resins are separated from the essential oils with which they are usually mixed by distillation with water, the resin remaining while the oil and water pass off; and from the gummy and mucilaginous matters by alcohol, which dissolves out the pure resins, which can be precipitated from their alco holic solution by the addition of water.
The resins are extensively employed in medicine; and in addition to the almost innumerable applications of caoutchouc and gutta-percha, various resins are of service in the preparation of varnishes, soaps, pigments, artificial light (resin-gas), etc.
Various fossil resins arc known, of which the most important is amber. Some chemists place bitumen and asphalt among this class; and among the fossil resins described by mineralogists may be mentioned fichtelite, partite, ozokeritc, schecrerite, xyloretin, etc.
The common resin, or rosin, of commerce exudes in a semi-fluid state from several species of pine, especially pines Wide, P. mitis, P. pelustris, and P. rigida of North America, P. pleader, P. pihea, and P. lurid(' of southern Europe, and P. sylvegris of northern Europe. The process of collecting it is very simple: a longitudinal slice of the bark and wood, about a foot in length, is taken off by means of mi axe with a curved blade, and at the bottom of the groove thus made a small piece of bent wood or thin metal, as tin or zinc, is driven into a curved cut made by one blow of the axe; this forms a sort of spout, which catches the liquid resin as it runs from the wound, and guides it into a small pot made of common clay burned. At certain periods these pots are emptied, and their contents put into casks for transport to the distilleries, where the volatile essential oil is removed from the resin. The resin thus procured is used very extensively in the manufacture of common yellow soap, also for sizing paper and various other purposes, including the preparation of ointments and plasters in pharmacy.
The other resins most generally known and used in Europe are anime (q.v.), copal (q.v.), dammar mastic (q.v.), sandarach (q.v.), frankincense (q.v.), lac (q.v.). In addition to these there are many which are of essential service in other countries, as the piny resin or' dhoop, obtained from vateria indica; black (laminar, obtained from cane 'slum strictum; soul resin, or dammar batu, from shone robenta—all of which serve many useful purposes in India, China, Japan, and other Asiatic countries. The forests of South America furnish many others.