The colour of wine vinegar is natural, but vinegar which is produced by the quick process is colourless or only faintly coloured. Since the public has a preference for vinegar which is brown in colour, the product of the quick process is coloured artificially, either by adding caramel or by preparing the weak spirit from malt which has been slightly charred in drying. Industrial Acetic Acid. The solutions of acetic acid dealt with above would be too dilute for any industrial purpose; moreover, the acid can be obtained much more cheaply by the distillation of wood. When wood is subjected to a high temperature, it is converted into charcoal and, at the same time, an inflammable gas, an acid liquid, and tar are given off, and can be collected in suitable vessels. The following table, on page 73, gives the relative amounts of the various substances obtained from wood by dry distillation. The quantities are those derived from one cord, that is, 125 cu. ft. _Charcoal _Alcohol _Calcium _Tar in _Wood oil _Turpentine in in acetate gallons._ in gallons._ bushels._ gallons._ in lbs._ gallons._ Hard 40-50 8-12 150-200 8-20 woods Resinous 25-40 2-4 50-100 30-60 30-60 Heavy woods woods 12-25 Light woods 2-10 Sawdust 25-35 2-4 45-75 The aqueous liquid that distils over contains methyl alcohol (wood spirit), acetone, and acetic acid. The crude mixture is known as pyroligneous acid. This is neutralized with milk of lime or soda ash, which converts acetic acid into calcium or sodium acetate, but has no action on the methyl alcohol and acetone which are also present. The mixture is then distilled, when methyl alcohol, acetone, and water pass over into the distillate, leaving the acetate in the retort. To obtain the free acid from the acetate, the latter is well dried and then distilled with concentrated sulphuric acid. Acetic acid, being the more volatile of the two acids, distils over, and is nearly pure. The method of removing the last traces of water depends upon the fact that acetic acid solidifies at 17° C. The acid, which is nearly, but not quite, free from water, is cooled until a portion solidifies. The part which still remains liquid is poured away, and the process is repeated until a residue is obtained which solidifies as a whole. This is glacial acetic acid, so called because it is a mass of glistening plates which look like newly-formed ice. The Acetates