The liquid principally employed to mix with these dry colours is _Linseed oil_. This is obtained by beating, pressing, or heating, from the seed of the flax plant, the _Linum usitatissimum_, which grows in most parts of Europe. This oil has so much _fatness_ or unctuousness, that it would dry with extreme slowness were not some further precautions taken. It is boiled with _litharge_ and _white vitriol_, in certain proportions, by which it has a drying quality imparted to it. _Nut oil_ is sometimes used in painting: this is procured from the kernels of walnuts, beech nuts, hazel nuts, and other kinds of nut, by a process similar to that by which linseed oil is obtained. _Oil of turpentine_, or _turps_, is largely used by painters, as it has a drying quality which counteracts the fatty nature of linseed oil, in combination with which it is generally used. It is obtained from a liquid or sap exuding from a species of pine tree, in North America: the sap is crude, or common turpentine; and by a process of distillation, the _oil of turpentine_ is obtained from it, leaving a substance behind which constitutes yellow resin. _Oil of spike_, _oil of lavender_, and _oil of poppies_, are sometimes used by the painter; but not very frequently, on account of their expense; they are vegetable preparations. _Pilchard oil_, (obtained from the fish,) _common tar_, _coal tar_, and _oil of tar_, are used occasionally for rough exterior work. _Varnish_, _size_, _beer_, _milk_, and one or two other liquids are used to a small extent in some processes to which the painter has to direct his attention. Varnishes are mixtures of various resinous bodies with spirit; and size is a jelly obtained by boiling parchment, leather, parings of hoofs, or of horn, or some similar animal substance, in water. Preparing the Paint. Such being the principal materials from which the painter prepares his paint, we proceed to speak of the mode by which he mixes them. The colours are mostly purchased in that form which is called _dry colours_, that is, in coarse powder or small lumps; and they have to be reduced to fine powder before they are mixed with the oils, &c. If they contain gritty particles of sand, &c., the colour is put into a tub or pan, and water thrown upon it, and mixed up with it. The gritty particles soon fall to the bottom, and the remainder is poured into another vessel, where, in a short time, the colouring substance falls to the bottom, and can be obtained by pouring off the water; after which the powder is dried. But if the substance is one which will dissolve in water, or if it is not very gritty, it is ground up to powder in a dry state.