PYROLIGNEOIIS ACID (LAT. Acidum pyrdignosum ; GER. H0/2.Sagre or ; FR. Acide pyro ligneux). The impact or pyroligneous aeid is obtained by the dry distillation of wood in close ovens. From the first distillation it is a dark, yellowish-brown liquid of varying strength, possessing an unpleasant clinging odour from the tarry compounds and various resinous matters with which it is more or leas impregnated. The manufacture is carried on extensively in various parts of this country, in France, Germany, and Belgium, for the production not only of the pyroligneous aeid, but also for the sake of the naphthas, charcoal, and various tar compounds which are obtained. Indeed the process is one in which all the products are utilized in a remarkable degree—from the cutting of the timber down to all the final issues.
Inasmuch as the item of carriage is an important consideration—owing to the bulk of several of the products, and to the necessity for a cheap and ample supply of timber isolating the works, and banishing them, as a rule, from the great centres of industry—it is advisable fur the manufacturer to select a site for his works within easy and convenient range of his staple raw material (wood), and not far removed from rail or water communication. Isolation from works of a similar character is necessary to prevent enhancement of the cost of the timber through undue competition. An ample supply of water for condensing purposes is also an essential, and if the source of the water be so situated that haud-labour and fuel can be saved by the employment of water-machinery, it is a, considerable advantage. The questions of isolation and ample supply of timber within easy range are too often lost sight of, the omission entailing serious loss in a trade where the turn over is comparatively small, and the saving of labour and prevention of competition important items. It should be appreciated that an ordinarily sized works, of say eight ovens, consuming 40 to 50 tons of wood per week, absorb over 100 acres of coppice in the year, and a coppice can only be advantageously cut once in every twenty years.
Large timber—indeed, as will be seen hereafter, almost any woody substance, except sueh as are decayed—may be used for distillation ; but, except perhaps in the case of beech, it is usual to take the " lop," or smaller branches, or " coppice" wood (small timber grown for the purpose). These coppices, which are generally a mixture of various woods (hazel, oak, beech, maple, (ke.), oceupy an immense acreage in many parts of the country, are sold by the landowner to the manufacturer at prices varying from 3/. to 15/. per acre, and are " cut " about every sixteen years. Sometimes younger growths are taken—some manufacturers maintain that they can get good yields from a seven or eight years' growth—but it is beyond doubt that the coppice does not reach its prime till it is about eigbteen years old. The amount of water in the wood is an important consideration in distilling, second only to that of the constitution of the woody fibre itself, inasmuch as it both takes up a portion of the heat, entailing a loss of fuel, and weakens the products of distillation. It will be readily apparent that the amount of water is greater in twigs and young shoots than in the more solid stem. It is also greater at the flow of the sap than when growth is less rapid, and hence, other things being equal, it is better to fell at the latter tirne.
With regard to the respective amounts of water in different woods, the fellowing table of Scliiibler and Hartig is worthy of note :— Hornbeam contains .. .. .. .. 18.6 per cent. of water. Willow ,, .. .. .. .. 26.0 „ 1/ Sycainore „ .. .. .. .. 27'0 „ Mountain Ash „ .. .. .. .. 28.3 „ „ Ash 28'7 „ „ Birch „ .. .. .. .. 30'8 „ „ Oak n • • - • • . • 34'7 „ „ White Fir „ .. .. .. .. 37.1 „ 71 Ilorso Chestnut „ .. .. .. .. 38..2 „ ,1 Pine „ Red Beech ,, •• - •• •• 39.7 „ /1 Alder „ .. .. .. .. 41.6 „ o Elm „ .. .. .. .. 44.5 „ o Rod Fir „ .. .. .. .. 45.2 „ 11 Lime „ .. .. • • .. 47.1 „ ).