Linoleic and homolinoleic acids are characteristic of linseed and other drying oils, while ricinoleic acid, which has properties very different from other acids, is found only in castor oil.

Both the oleic and linoleic series of acids are monobasic, like the stearic series, and combine with potash and soda to form soaps which are rather more soluble in water than the soaps made from the stearic acids.

Glycerine, the sweet spirit of oils, is a water-white, very viscid liquid, quite odourless, but possessing a sweet, though metallic, sort of taste. It is heavy, having, when pure, a specific gravity of 1.270. When heated it volatilises with difficulty, being slightly decomposed during the operation. It will only burn when heated, and then with a smoky flame having a small amount of luminosity.

It has a great affinity for water, with which it mixes in all proportions, and which it absorbs from the atmosphere in no small proportions, being strongly hygroscopic. On this account glycerine gradually becomes weaker when exposed to the air. It is soluble in alcohol.

Glycerine is a compound of the basic organic radicle glyceryl (CH), with three equivalents of hydroxyl, OH, and has the JOH

formula C, H, OH It possesses alcoholic properties, and is

3

он

capable of combining with acids; with monobasic acids it requires three equivalents to form saturated salts, and hence is capable of forming three different compounds with such acids; thus, with oleic acid it forms

in which one, two, or three equivalents of hydroxyl are replaced by one, two, or three equivalents of oleic acid. These compounds can be formed by the direct union of oleic acid and glycerine, and it is of interest to note that the triolein so made is indistinguishable from the olein which is present in oils.

When oils are boiled with solutions of caustic soda or potash they are decomposed. A compound of alkali and fat acid is formed while glycerine is liberated. This reaction is shown in the following equation:

C16 H27 02

но

C3 H5 C16 H27 O2 + 3 Na O H = 3 Na C16 H27 Og + C3 HS HO

C16 H27 02

Linolein.

Sodium
Hydroxide.

Sodium
Linoleate.

но Glycerine.

This reaction is known as saponification, because the alkaline compound obtained forms the familiar article, soap.

As stated above, the oils are divisible into two groups-nondrying and drying oils. The former group is by far the larger of the two, but the oils in it are of no use to the painter. The latter group, which is the one which will be dealt with in this chapter, is small in number, but it contains oils of great importance to the painter.

THE DRYING OILS.

There are but few oils, compared with the great number known, that can be used for painting on account of their possessing the essential property of becoming hard or drying when exposed to the air. The best drying oils are obtained from vegetable sources, although one or two are obtained from animal sources. The following list includes all that can be included within this group :

A few other oils, such as niger seed oil, cress seed oil, grape seed oil, cotton seed oil, possess weak drying properties, but these are of much too weak a character to permit of their being used as paint oils. Rosin oil is also offered as a paint oil, but it is not a good drying oil; it will be dealt with in detail further on.

The most important of this group of oils is linseed oil, which is the painter's oil par excellence; the others are only used in painting on a comparatively small scale, and mostly by artists, not because they are any better drying oils than linseed, but because they have a paler colour, and, therefore, do not affect the tints of the colours quite so much-a matter of some importance when delicate tints have to be painted. For housedecorating purposes no other oil is used, because no other is so cheap or abundant.

LINSEED OIL.-This oil is obtained from linseed-i.e., the seeds of the flax plant, Linum usitatissimum-which is cultivated both for its seed and for its fibre, which latter is spun and woven into linen. Ireland, England, Holland, Germany, Russia, America, Canada, and India are noted for the cultivation of the flax plant, which will grow anywhere where the climate is not too hot, but a cold or temperate climate suits the plant best.

Linseed is a small seed of a flat oval shape, somewhat pointed at one end; it is lustrous, and of a pale brown colour. It varies a little in shape and colour, according to the locality in which it is grown, and an expert can tell by inspection without much error from whence a particular parcel of seed has come. It is exported in large quantities from Riga, Libau, Taganrog, and other Russian ports, and from Calcutta in India. Smaller quantities come from other places, but these occupy only a minor position compared with the seed coming from the places named above. Three qualities of linseed are recognised in the tradeBaltic seed, which comes from Riga and other ports on the Baltic coast of Russia, and is the seed of flax grown in the north of Russia; Black Sea seed, coming from Libau, Odessa, and other ports on the Black Sea, and which is the seed of flax grown in Southern Russia; and East India seed, which is exported from Calcutta. The Baltic seed yields the best and most valuable oil, that from Black Sea seed is next in quality, while East India seed gives oil of inferior quality.

The oil obtained from seed grown in America, Canada, and other places, is mostly used locally, and very little finds its way to the English market. The seed imported is rarely free from other seeds such as those of hemp and rape.

Extraction of Linseed Oil.-The oil is obtained from linseed and other seeds by a process of pressing; but, before being pressed, the seeds undergo some preliminary treatment, with the object of facilitating the process of oil extraction. Naturally the process of extracting oil has undergone many changes during the last fifty years, and for the purpose of this work, it will be sufficient if the modern systems only are described in outline; for further details, the companion work on Lubricating Oils in this series of text books may be consulted.

Two systems of oil extraction are used-viz., the English and the Anglo-American; these do not differ very much from one another.

The English system includes five operations:-1st, crushing; 2nd, grinding; 3rd, heating; 4th, pressing; 5th, refining.

1st, Crushing.-Prior to this, however, the seed is sifted

through sieves, with the object of separating out as much dirt and foreign seed as possible; this is a preparatory operation, common and essential to all systems of oil pressing, as the presence of much oil from other seeds is apt to spoil the linseed oil. As the majority of the foreign seeds found in the crude linseed are rather smaller than linseed itself, nearly all are separable by using sieves of a certain gauge of mesh.

The sifted seed is now passed through a crushing machine, which consists of a pair of heavy wheels placed horizontally in a strong frame (Fig. 55). One of these wheels is 4 feet in diameter, and is driven by the one to which power is applied; the other wheel, one foot in diameter, is generally driven by friction from the larger wheel, but in some makes of the mills by gearing; by means of a system of screws and springs in

connection with the bearings of the small wheel, a regulated pressure can be brought to bear on any seed which passes between the two wheels. The large wheel is driven at about fifty-six revolutions per minute. A hopper placed above the wheels supplies the seed in a regular manner, which is crushed in passing between the wheels, and falls into the receptacle

placed underneath the machine for the purpose of collecting the crushed seed. Attached to each wheel is a scraper for keeping the surface of the wheel free from crushed seed.

2nd, Grinding.-From the crushing machine the seed passes to the grinding mills, which are of the edge-runner type (Fig. 56), and consists of a pair of large stones, about 7 feet in diameter,