Another important distillation process is the so-called dry distillation, in which organic sub stances are decomposed into solid, fluid, and gaseous products by means of high temperatures and with the exclusion of air. As a rule these products, called empyreumatic, are formed simultaneously, but are separately collected and treated. An example of dry distillation, which is also called destructive distillation, as it is accompanied by chemical changes, is the distil lation of coal for the manufacture of illuminat ing gas. Ammonia-water (in the raw condition, tar-water, which contains cyanides and sulpho cyanides in connection with ammonia), semi fluid, viscous, oily, and very valuable products, as, for example, tar-oil, which is extensively used in the manufacture of aniline colors, anti septics, and medicinal preparations, are obtained as by-products, while coke is the residue of the coal distillation.
The distillation of wood is conducted in the case of resinous woods by steam, and with hard woods by dry distillation. The former is pre ferred in the manufacture of turpentine, be cause it avoids drawing the tar from the wood. The retort is filled with pine wood chips, and steam is turned in with a rush, so as to heat the whole as quickly as possible, the pressure not to exceed ten pounds to the square inch. The contents of the retort are agitated by a mechanical stirrer, and the process continues as long as a paying quantity of oil comes over. With the turpentine is a small proportion of ethers and aldehydes, and some resinous mat ters. The turpentine is redistilled for the mar ket, leaving as a residue, resin oil. The resin (colophony) remaining in the retort is distilled in an iron still by the dry process and yields resin spirit and resin oil, and resin tallow. The resin oils are rectified by the distillation process, followed by a chemical process. In the distilla tion of hard woods without steam, the products are wood vinegar (pyroligneous liquor) 42 to 45 per cent — equal to acetic acid 7 to 9 per cent; tar 8 to 9 per cent; and charcoal 22 to 26 per cent. Further distillation of these pri mary products results in the gaining of a great number of substances from the tar, particularly paraffins, phenols and esters such as paracresol, guaiacol (from beechwood tar), kreosol, pyro gallic esters, fatty acids and their esters, light tar oils and heavy tar oils, and pitch. From
the pyroligneous liquor is obtained acetic acid, up to 10 per cent; wood alcohol up to two per cent; and small quantities of propionic, butyric, formic and other acids, methyl acetate, allyl alcohol, furfurol, phenols, amines, ketones, etc. The commercial manufacture of wood alcohol, however, is accomplished by treating wood with concentrated sulphuric acid, thus transforming its cellulose to a form of sugar which is then fermented and the mash distilled.
Resins of all sorts are distilled before being of value for varnish making. The process used is generally by superheated steam under pressure.
Ether is obtained by distilling sulphuric acid and alcohol together in a steam-heated lead lined retort, and passing the vapors through a solution of sodium carbonate, or milk of lime, to remove all traces of acid. A second distilla tion removes the alcohol, and the ether is recti fied in a columnar still.
Besides these above-mentioned industries, many others are also based on the distillatory process.
A special branch of distillation is that in which a solid is vaporized and by condensation directly passes from the gaseous to the original solid usually in a crystalline form. This proc ess is called sublimation, and is mostly used to purify volatile solids, as for example, sal ammoniac (chloride of ammonia), benzoic acid, camphor and indigo. The most interesting fact in sublimation is that the solid does not pass through a liquid state, but at once to the gaseous form. The reverse process also takes place in like manner, namely, the condensation from the gaseous state back directly to the solid condition.
Brachvogel J. K., 'Indus trial Alcohol> (New York 1907) ; Dumesny, P. and Noyer, J., 'Wood Products, Distillates and Extracts> (London 1908) ; Lewes, V. B., 'The Carbonization of Coal> (New York 1912); Lunge, G., (Coal Tar and Ammonia' (New York 1909); Martin, G. 'Sulphuric Acid and Sulphur Products' (London 1916) ; Schweizer, V., 'Distillation of Resins' (New York n. d.); Walter, E., 'Manual for the Essence Industry> (New York 1916) ; Wright, F. R., (Distillation of Alcohol> (New York 1906) ; United States Chemistry Bureau Circular 36.