and is in contact, therefore, with a much larger quantity of oxygen.

4th. Turpentine does not act as a drier—that is, as a carrier of oxygen to the oil.

5th. Different white pigments behave differently when drying; the more powerfully basic the properties of the pigment, the more powerful is its action as a drier. Lead oxide and white

lead (basic lead carbonate) combine more easily with the acids of linseed oil than zinc oxide does. But zinc oxide dries better than antimony oxide, because it is a stronger base, while arseniate of tin has no basic properties, therefore does not act as a drier.

6th. Indifferent substances-that is to say, those without chemical action on oil, such as lamp black, barytes, sulphate of lead-cannot act as "driers."

Properties of Boiled Oil.-Boiled oil is a slightly viscid oil of a reddish colour, varying a little in depth of colour according to the temperature, and the length of time it has been heated in the process of boiling. Its odour is peculiar, and its taste, which is rather acrid, somewhat characteristic. In specific gravity it varies a good deal, but the average is about 0·945; some samples will reach 0.950, while others may be as low as 0.940. Boiled oil is soluble in turpentine, petroleum spirit, shale spirit, benzene, carbon bisulphide and other similar solvents. When boiled with caustic soda or caustic potash it is saponified almost completely; there is usually a small trace of unsaponifiable hydrocarbon oil formed by the decomposition of the oil during the process of boiling.

When exposed to the air in thin layers it dries much more rapidly than raw linseed oil, and leaves behind a hard, lustrous coat; it is this property which makes boiled oil of so much use to the painter; yet it does not do to use boiled oil alone in the making of paints, because the coat which it leaves is too hard and rather liable to crack on exposure to the air; raw linseed oil is always added, as, by leaving a more elastic coat, it prevents this bad fault of boiled oil from showing itself.

Adulteration of Linseed Oil.-Both the raw and boiled linseed oils are frequently adulterated; (substitutes for boiled oil will be described more fully later on); the principal adulterants used are mineral and rosin oils. Other fatty oils, such as cotton seed, niger seed, and whale oils, are sometimes used; but, as linseed oil is cheap, the small gain arising from their use does not compensate for the probable loss of custom

which must ensue if the adulteration be found out; while the great difference in the cost of linseed and mineral oils is a strong inducement for adulterating with the latter.

For the purpose of detecting adulteration the following tests may be applied :

1. Specific Gravity.-For raw linseed oil this should be about 0.932; if less than 0.930, adulteration with fish, seed, or mineral oils would be indicated; while if the gravity exceeds 0.937, then admixture with rosin oil is very likely. The specific gravity of boiled oil averages about 0.945; if much heavier than this it is quite probable that rosin oil has been mixed with the oil; while if below 0.940, then other fatty and mineral oils may be looked for. 2. Flash Point.-Linseed oil, whether raw or boiled, flashes at about 470° F. Other fatty oils flash at about the same temperature. Rosin oil flashes at from 300° to 330° F., and during the process of testing a strong odour of rosin would be given off. Mineral oils, such as would be used to adulterate linseed oil, will flash at from 380° to 420° F., so that the flash point is one of the best tests for detecting the adulteration of linseed oil with mineral or rosin oils.

3. Proportion of Mineral or Rosin Oils in Linseed Oil.-To determine the proportion of mineral or rosin oils, in adulterated linseed oils, place 20 grammes in a beaker with a little water and alcohol; then add some caustic soda and boil for some time, stirring at intervals; the linseed oil becomes saponified, while the adulterants are not acted on; after about an hour's boil, the mass is allowed to cool a little; then it is poured into a separating funnel and some petroleum ether is added, which will take up the mineral oil and form a layer on the top of the aqueous layer; after allowing the two layers to separate completely, the bottom layer is run off, and the top layer is washed quite free from all traces of the soap formed by the action of the alkali on the linseed oil, by several treatments with warm water. The ethereal layer is then run into a weighed glass, the ether evaporated off, and the residue of mineral oil weighed. Whether the residue is mineral or rosin oil must be judged from the nature of the residual oil after evaporating off the ether; if this is heavy and viscid, and smells of rosin when heated, then rosin oil is present; if the residual oil is light, then mineral oil is present.*

* A simple test for the presence of rosin or of mineral oils in either raw or boiled linseed oil, is to boil a small quantity with an alcoholic solution of caustic potash until it is completely saponified, then pour the solution into water; if the oil be pure, a clear mixture will be obtained; if it contains either of the oils named it will be cloudy and turbid.

4. Cotton and other Fat Oils in Linseed Oil.-The detection of cotton seed, niger seed, or other fat oils in linseed oil is much more difficult, but much valuable information on this point will be gained by noticing the behaviour of the oil with strong sulphuric acid, the character of the mass formed, and the temperature which the mixture of acid and oil attains. The character of the soap formed on boiling the oil with caustic soda, the appearance, melting point, and combining equivalent of the fatty acids which may be obtained from the soap so formed are also valuable indications of the character of the fatty oil adulterants.

5. Driers in Boiled Oil.-About 25 grammes are boiled with a little dilute hydrochloric acid, with constant stirring, for about half an hour; the mass is allowed to stand to separate; the bottom acid layer contains the driers added during the boiling of the oil, this is run off and tested in the usual way; then the oil is boiled with caustic soda until it is saponified, then the mass is treated in the separating funnel, as described above, to separate the mineral or rosin oil used to adulterate the boiled oil. The aqueous layer which has been run off may be acidified, and the acids obtained tested for rosin by Gladding's test.

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BOILED OIL SUBSTITUTES.-Many substitutes offered for boiled oil, some of which have been patented. In composition they vary greatly, and it is not possible to do more than briefly indicate their general features. Some are mixtures of boiled oil, rosin, turpentine, rosin oil; others more closely approach an oil varnish in composition, being made by melting rosin, then mixing it with hot oil and thinning down with rosin spirit. Some are made by preparing a compound of lime or alkali with rosin or other resinous products, and dissolving this in oil and rosin spirit or turpentine.

The quality of these products varies very much. None of them are equal to good boiled oil, although one or two very nearly approach it; others are but inferior substitutes, and cannot be recommended even for inferior work. It is not possible to deal more particularly with these boiled oil substitutes in this book.

POPPY OIL.-This oil is obtained from the seeds of the poppy (Papaver somniferum) by pressure, or it may be extracted by means of solvents. This oil, although a very good drying oil, is not largely used, chiefly because its price does not allow it to compete with linseed oil; artists make use of it on account of its paleness in colour not interfering so much with pale tints as linseed oil does, its price not being so much an object with them as it is with house painters.

Poppy oil is usually of a pale straw colour, very limpid, has little or no odour when fresh, and a pleasant taste; the oil is free from the narcotic properties for which the plant itself is famous. In specific gravity it ranges from 0.924 to 0.927. It solidifies at -18° C. It is soluble in about four times its volume of boiling alcohol and twenty-five times its volume of cold alcohol. Mixed with strong sulphuric acid (Maumene's test), the rise in temperature is about 88° to 90° C. It takes about 19 per cent. of caustic potash (KOH) to saponify it, and it absorbs about 134 to 137 per cent. of iodine.

HEMPSEED OIL.-The hemp plant (Cannabis sativa) yields a roundish greenish-grey seed, very familiar to lovers of canaries, from which, on expression, an oil is obtained that is used for painting. The yield of oil varies from 15 to 25 per cent.

Hempseed oil when fresh has a greenish-yellow tint, but on keeping it slowly turns to a brownish-yellow; its odour and taste are rather unpleasant. Its specific gravity ranges from 0.925 to 0.931. It becomes turbid at a temperature of 15° C., but does not set completely solid until a temperature of 25° C. is attained. Strong sulphuric acid has a vigorous action on it, the increase in temperature being about 100° C. It absorbs from 143 to 144 per cent. of its weight of iodine, which indicates that it contains a large proportion of linoleic acid (linolic and linolenic acids), and shows that its drying properties must be good.

In this country hempseed oil is rarely used as a paint oil, its price being against it; still, it has been mixed with linseed oil, and it is difficult to obtain the latter free from it, owing to the Russian linseed growers mixing hempseed with the linseed. In Russia, and other places where hempseed is grown, the oil is used rather largely for painting.

WALNUT OIL.-The common walnut, the fruit of the walnut tree (Juglans regia) contains about 50 per cent. of its weight of an oil possessing drying properties. The process of extraction of this oil is as follows:

The nuts are collected and placed in heaps for a period of about three months, when they begin to decompose; they are then crushed and pressed; this gives "virgin nut oil," often used as a food oil as well as a paint oil. The nuts still contain some oil, which is extracted by grinding the cake with hot water and again subjecting it to pressure; the oil so got is known as drawn nut oil."

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Walnut oil is usually of a pale yellowish-green tint, but it can be prepared almost colourless from fresh kernels. The specific gravity varies from 0.925 to 0.927; it begins to be turbid at a

temperature of -15° C., but becomes solid only when at a temperature of 27.5° C. Strong sulphuric acid causes the evolution of some heat, the increase in temperature being 101° to 103° C. It will absorb about 144 per cent. of iodine, pointing to its containing linoleic (linolic and linolenic acids) in large proportion.

It is a powerful drying oil; some authorities say that it is superior to linseed oil in this respect; at all events it is quite equal to it in drying power. It is chiefly used by artists, as it is pale in colour, and can, by bleaching, be obtained almost colourless. Its greater cost prevents its coming into extensive use as a substitute for linseed oil in house painting.

CHINESE WOOD OIL, or TÙNG OIL, is obtained by cold pressure in a somewhat primitive manner by the use of wooden presses from the seeds of the Tung tree, Elaococca vernica, a member of the Euphorbiacea, a group of plants which grow very extensively on the banks of the Yangtsze river in China. The seeds yield 35 per cent. of their weight of the oil. The oil, when of good quality, is pale amber in colour, somewhat dull in appearance, but lacking that brightness which is seen in rape or cottonseed oil. It has a peculiar and characteristic nutty odour and unpleasant taste. Its specific gravity at 60° F. is 0.940; at 212° F., 0·8871. It is rather viscous, being slightly more so than linseed oil, the viscosity at various temperatures being as follows:-At 70° F., 150; at 120° F., 98; at 150° F., 52; and at 212° F., 22. Mixed with strong sulphuric acid it forms brownish clots, and gives rise to considerable increase of temperature, a mixture of 20 parts oil to 8 parts strong acid rising to 182° F. It takes 17.78 per cent. of caustic potash to saponify it; this points to the oil containing acids of high molecular weight. It contains a small quantity of free acid, about 1 to 3 per cent.

The chemical composition of this oil has not as yet been fully worked out. To one of its constituents the name of Elæomargaric acid has been given.

It absorbs 93 per cent. of its weight of bromine, or 120 per cent. of iodine.

There are both soluble and insoluble fatty acids in this oil; subjected to the Reichert distillation test, 10 grammes of the oil give a distillate which has an acidity equal to 16-2 c.c. decinormal caustic soda, while there are 88.7 to 90 per cent. of insoluble fatty acids.

Poured in a thin film over a glass plate it takes about two days to dry, being, therefore, quicker in its drying power than raw linseed oil, which takes about three days to dry. The dry