PYROLIG'NEOUS ACID, or WOOD VINEGAR, a crude commercial from of acetic acid (q.v.). It is made by the destructive distillation of wood, and contains, besides acetic acid, tar and other products, which have to he removed if it is required in a very pure state. Generally, it is obtained in Britain from oak branches, which, after being stripped of their bark, are too small for timber purposes. .These are cut into short billets, which are placed in cast-iron retorts, and a sufficient heat applied to drive off the volatile con stituents and carbonize the wood. The best woods for the distiller are " hard" woods, although all will yield it. This will be seen from the following table, which partly summarizes the experiments of Stolze: 100 Parts of Dried Wood give Crude Pyro- Pure Hydrated ligneous Acid. Acetic Acid.
(Beta& alba) • 45.0 4.47 _Beech (Rios syleatica). . . 44.0 4.29 Oak (Quereus mbar) . 43.0 3.88 Ash (Ft.a.vinus excelsior). . . . ....... . . . .. 46.8 3.72 White Poplar (Populus alba) .... 45.8 3.23 Bird (Prunus paclus) 47.3 2.92 Juniper (Iuniperus communis) 45.8 2.34 Spruce fir (Pines abies) 41.2 2.16 Scotch fir (Pines sylrestris) 42.4 2.14 Quick distillation is always found to be much more productive than slow, and the acid is also freer from impurities; for the slower the process, the thicker and darker is the tarry matter. Hence two separate plans have been invented, one by Mr. Halliday, and the other by Mr. W. H. Bowers, both of Manchester, in which sawdust, chips, shavings, and spent dye-woods are used. In Mr. Halliday's plan, the retort is a long tube, with the fire acting along its entire length; inside is an Archie:reclean screw, worked by machinery, which passes the sawdust or other material slowly from the commence ment to the end, where, by particular contrivance, it falls out in the state of thoroughly carbonized wood. It is supplied by means of a hopper. The volatile matters pass up
an outlet-pipe in the upper part of the tubular retort. In Mr. Bowers's plan, the prin ciple is similar, though differently.carried out. The sawdust is fed through a hopper which is always kept well supplied, so that, by the pressure of the supply, the escape of vapor may be prevented. An endless chain is worked over four rollers by a small steam engine, and carries the materials from the hopper by means of projections on the chain along the lower side of the retort, so as to bring them in contact with the furnace. By the time the material reaches the bottom, all the volatile matters have been vaporized, and have passed up into the condenser and the carbonized material falls into a cistern of water into which the open end of the retort dips, the water closing it sufficiently. One of these retorts will yield about 200 gallons per day of pyroligneous acid. This acid is of great use in the arts, especially in making the acetates used by dyers and calico printers; and it is also, when very carefully purified and properly diluted with water, used extensively as a substitute for common vinegar in pickling, and even for table use.