The last distillate, when a small portion of it is shaken up with its own bulk of benzene, should mix perfectly with the latter, causing no turbidity. But it should be borne in mind that absolute alcohol is a very hygroscopic liquid, greedily absorbing water from the air; it must, therefore, be kept in well-stoppered bottles, filled almost completely. In absolute alcohol some of the more intractable resins, even some kinds of copal, readily dissolve. The specific gravity of absolute alcohol at 15° C. is .794, while, if it contains but 1 per cent. of water, its specific gravity is distinctly higher, namely, .797.

Benzene is employed not only as a solvent, but as a diluent of the medium or oil employed in painting. It is obtained from the lighter naphtha separated in the fractional distillation of coal-tar. The benzene (also called benzol) of commerce is rarely pure. The presence of small quantities of higher hydrocarbons of the same series is of little moment, but it also contains about one half per cent. of a sulphur compound called thiophene (C4H4S), to which the offensive odour of ordinary benzene is partly due. Thiophene is, however, much more soluble in cold oil of vitriol than is benzene, and may be removed by several treatments of the benzene with small quantities of this powerful acid. Benzene thus purified can now be purchased. Benzene is a mobile liquid, not miscible with water, but dissolving readily in all proportions in most if not all of the liquids now being described. It dissolves oils and very many of the harder as well as all the softer resins.

Toluene, commercially known as toluol, much resembles benzene, and may be used for the same purposes, although it is less volatile. Commercial toluene has a disagreeable smell, arising from the presence of a sulphur compound (thiotolene), which is more difficult to remove from the liquid than the thiophene from benzene.

Toluene of good quality and at a moderate price may be obtained from Kahlbaum of Berlin. It constitutes a useful diluent and solvent when used with the spirit-fresco medium.

Epichlorhydrin has been employed as a solvent for some of the copals and other intractable resins in the manufacture of varnishes. Without further trials it would be unwise to employ this liquid in artistic painting, yet the resins dissolved in it seem to retain much of their original toughness and hardness. The same statement may be made in reference to several other similar chlorine compounds, which possess the merits of cheapness and non-inflammability.

Antyl alcohol, the chief constituent of fusel-oil, is used by picture-cleaners for the removal of discoloured varnish. Its vapour is suffocating and even poisonous.

Pinene, Sylvestrene, Limonene, and Dipentene, with several other similar compounds, are the main constituents of the various liquids to which the ordinary name of turpentine, or, rather, spirit or oil of turpentine, is applied. All these liquids are hydrocarbons, having the same composition in 100 parts, expressed by the empirical formula C10H16. But these liquids - of which about ten are known - differ from one another in some of their chemical and physical characters, such as oxidizability, boiling-point, specific gravity, and action on light. The extreme importance of turpentine in the process of oil-painting, and in the manufacture of varnishes, warrants a somewhat full consideration of its several constituents.

Turpentine, properly so called, is not a liquid, but the solid or semi-solid resinous secretion of many trees, chiefly coniferous. Some exudes naturally, but much more is obtained by artificial incisions. It consists of a mixture of one or more true resins and resinous acids in which oxygen is present, with one or more liquid hydrocarbons which contain (as the name imports) nothing but carbon and hydrogen, and therefore no oxygen. These hydrocarbons are called in chemical language terpenes, a term by which they will be designated henceforth in the present chapter. On distilling the crude turpentine or resins alone or with water, or in a current of steam, the terpenes distil over while the solid part remains behind; this, on fusion, is called rosin or colophony. It need not be further considered, as it is of no value in painting, being friable and more or less strongly coloured; it is, however, employed in making certain 'dryers,' known as resinates (or better, rosinates), containing cobalt, manganese, etc. We confine our attention, therefore, to the distillate or terpenes.

It should be added, however, that the leaves, cones, and other parts of many coniferous trees, themselves yield various terpenes when submitted to distillation, and that many of the volatile or essential oils of aromatic plants other than conifers contain or consist of terpenes. The oils expressed from the rinds of lemons and oranges afford illustrations of this remark.

Terpenes differ from one another in several obvious and in several obscure ways. Even now the chemistry of these liquids is not by any means clearly and completely unravelled. We need not here concern ourselves with those minute differences in chemical and physical properties by which the identity of individual terpenes is established, but may confine our attention to their most salient characteristics. Of these none is more important than the behaviour of terpenes with regard to atmospheric oxygen. Some of these liquids absorb oxygen readily, and to a large extent, from the air, becoming thereby resinified - in fact, they thus yield sticky, resinous, semisolid bodies, closely resembling the crude turpentine from which they have been prepared. Everyone who has had occasion to use spirit of turpentine frequently must have noticed the production of a sticky substance about the neck of the bottle in which this liquid has been kept. Moreover, the spirit of turpentine itself will often have been noticed to have become cloudy, viscid, or almost solid, especially if it has been contained in a bottle frequently opened, and not quite full.