Much linseed now comes from the Argentine, Canada, and the United States, as well as from India and Russia.

The percentage of oil in linseed varies between 28 and 45: by cold-pressure 20 per cent. is the average yield; by hot-pressure, 27 per cent.; by extraction with carbon disulphide, 33 per cent. The linseed oil in common use by artists is hot-pressed oil, and is very rarely, if ever, obtained from absolutely pure seed. The seed should be kept three months before it is pressed. The expressed oil should be exposed to light in covered glass vessels or tanks, and kept at a temperature of 212º F. for some time. It thus loses colour and becomes clear, a slimy deposit containing mucilage, albuminoid matter, and traces of a cyanogenetic glucoside, being formed. When thus bleached and clarified, the oil should be preserved in corked bottles filled quite full; the longer it is kept, the better it becomes for painting, provided the access of air is prevented. The specific gravity of good linseed oil varies very little. At 60° F. (15.6° C.) it is .935; a bottle which will hold 1,000 grains of water at this temperature will therefore hold but 935 grains of linseed oil. It expands considerably with heat, its specific gravity at 50º C. being .913 only. One part of linseed oil requires 36 parts of cold absolute alcohol for solution, but only 4 parts of boiling alcohol.

It may be purified by solution in boiling alcohol or in petroleum ether. Other methods of purification are generally employed. Amongst these ma>be named the following: Filtration through felt or carded cotton and charcoal, and then through pyrolusite; contact for some weeks with 3 per cent. of a mixture of equal parts of kaolin and aluminium hydrate, both these compounds having been previously dried at about 50° C.; agitation with a solution of common salt, followed by washing with water, and drying by a heat of 220° F.; treatment with one four-hundredth part of oil of vitriol, addition of hot water, washing, and drying. Various other processes and reagents have been employed for purifying and bleaching linseed oil. Aqueous solutions of sulphurous acid, green vitriol, potassium permanganate, potassium bichromate, and peroxide of hydrogen may be included in this list. The addition of 1 per cent. of oil of turpentine to the oil, and then passing a mixture of air and steam through it, has also been tried. Whatever process be adopted, no acid, saline matter, or moisture must be left in the oil.

The general and usual result of all the very different kinds of treatment to which linseed oil is subjected, in the above-named and in many other processes, seems to be the more or less complete removal of impurities. The effect on the properties of the purified oil is chiefly seen in its greatly increased rate of absorbing oxygen and consequent hardening.

* Occasionally these weed-seeds give up, under pressure, certain matters which deepen the colour of the expressed oil somewhat..

The chemical composition of linseed oil may now engage our attention. Its ultimate analysis shows it to vary according to the method of extraction adopted, cold-pressed oil containing about 78 per cent. of carbon, 11 per cent. of hydrogen, and 11 per cent. of oxygen; while the hot-pressed oil contains nearly 3 per cent. less carbon, and nearly 3 per cent. more oxygen - linseed oil, extracted by carbon disulphide, is still poorer in carbon, and richer in oxygen. It appears that linseed oil consists chiefly of three glycerides, called, respectively, linolein, linolenin, and olein. A small, but variable, amount of free fatty acids, such as palmitic and arachidic, is also present. The empirical formulæ of the three fatty acids of the above-named glycerides are, respectively:

Linolein, which is present in linseed oil to the extent of about 20 per cent., is the glyceride of linoleic acid, and has the formula (C18H31O)3,C3H5,O3; or, as it may be written, C3H5(O,C18H31O)3. The relation of this glyceride to glycerin may be seen when the latter body is expressed by the formula, C3H5(OH)3. It is probable that the other main constituent of the oil - linolenin - is a similarly constituted glyceride, and that it closely resembles linolein in physical and chemical properties. When 100 parts of linseed oil are saponified by an alkali, they yield from 9.4 to 10 parts of glycerin.

The most important chemical property of linseed oil, from a painter's standpoint, is its behaviour with oxygen. Under certain circumstances, it absorbs oxygen to the extent of 13 or even 14 per cent. of its weight, becoming converted into a mixture of substances for which it is convenient to retain the old name linoxine. Linoxine is solid, and not liquid; it is far less soluble than linseed oil in any solvent, and in many liquids it is insoluble. Linoxine is, moreover, denser than the original oil; 100 grains of linseed oil produce about 109 or no grains of linoxine. Notwithstanding the greater density of linoxine, when compared with the original oil, its formation is attended by a considerable expansion. In consequence, a layer of raw linseed oil spread upon glass becomes wrinkled during the drying and oxidizing process. During the oxidation of linseed oil, the small quantity of olein it contains remains unoxidized - its presence confers elasticity upon the product.

¶ The incidents associated with the hardening or solidifying of drying oils have always been an attractive study, but it is only through the chemico-physical researches of recent years that their true nature has been made clear. The most important points will now be briefly explained. This hardening depends, as before stated, on a process of oxidation - that is, on the absorption of free oxygen from the atmosphere. During this process carbon dioxide and other volatile organic compounds are formed and given off, while simultaneously there are produced solid, nonvolatile bodies which constitute the dried and hardened oil. These solids then, in their turn, by a further and very slow oxidation, yield other volatile products. While in the first stage, the gain in weight of the oil, due to the absorption of oxygen, far more than compensates for the loss which arises from the escape of volatile matters, in the second stage there is a distinct diminution in bulk and in weight, while the residue acquires a deepening brown hue.