134 MANUFACTURE OF WOOD-GAS: DISTILLED PRODUCTS. 



Oilier distlUed products of wood. — Keferring to special treatises, aocl 

 especially to Viucent's above quoted, for a particular account of methods, 

 we can here only enumerate some of the principal liquid and crystal- 

 line products to be obtained from woods, chiefly from the oak, beech, 

 hornbeam, and other hard woods — not earlier than 20 years of age, and 

 cut in the winter-time. 



ucts, which causes a considerable cooling of the retorts, and therefore it becomes nec- 

 essary to apply a large amount of heat -within a very short time. The material of 

 which the retorts are made should therefore have a greater power of conducting heat 

 than those used with coal. It is therefore best that they should be made of cast iron. 



The form of the retorts is generally the same as that for coal. The charge being 50 

 kilograms (llOJ pounds) of dry wood, they should be 11.8 to 13.8 inches high, 22 to 

 23J- inches wide, and 102J- to 106^ inches long. For a charge of 75 kilograms (165J 

 pounds), we use retorts 17| inches high, 25.6 inches wide, and 106|- inches long. 



A retort of the first of these sizes will yield from 200 to 240 meters (7,003 to 8,475 

 cubic feet) in 24 hours, while one of the second size should give 250 to 300 meters (8,629 

 to 10,594 cubic feet) in the same time. 



From what has been said it will be inferred that the furnaces should be like those 

 employed' for coal, but with a larger amount of grate-surface. In regular working 

 order, an operation lasts an hour and a half, and about one-half of the whole volume 

 of gas is disengaged during the lirst half-hour of the distillation. When the operation 

 is done, the retorts are opened, and the charcoal is drawn out into sheet-iron extin- 

 guishers, which are at once closed and luted, and then left to cool. 



As the gases come out of the retorts, they pass into a cask-shaped vessel, and then 

 to the cooling apparatus ; but because of the very rapid distillation of the wood these 

 parts of the apparatus should have a very considerable diameter, and should be cooled 

 more energetically than those used in making coal-gas. They generally give a ^ 

 shaped form to the section of the cask, to facilitate the joining of the plunging-pipes 

 coming from the retorts, and place it in a sheet-iron vat that is cooled by a stream of 

 cold water. Moreover, as the products are strongly corrosive from the acetic acid, it 

 is well to make the coo'ing apparatus of copper, because sheet iron, or even cast iron, 

 would be otherwise rapidly consumed. In the receiving-cask the tar and pyroligueous 

 acid are condensed, and fiow out in a steady stream. The gas on coming out of this 

 receiver passes into coolers and washers analogous to those used for coal-gas, and then 

 into pits of lime-purifiers. 



Wood-gas contains a quantity of carbonic-acid gas, varying from a quarter to a fifth 

 part of the whole volume, from which it will be inferred that the hydrated alkalies 

 are the only agents that can be advantageously employed for the rapid purification of 

 such a mixture. They therefore pass the gas through purifiers charged with quicklime, 

 mixed with light substances for dividing it, such as tan-bark, mosses, or sawdust, 

 placed in layers two or three inches thick, taking care to make the gas pass up evenly 

 through the layers of half lime. In practice, they use 100 to 110 kilograms (220^ to 

 242^ pounds) of lime for purifying 100 cubic meters (3,531 cubic feet) of gas, with 10 

 square meters (107.6 S(iuare feet) of the loose materials, and still there will often re- 

 main from one to two per cent, of carbonic-acid gas in the purified gas. The lime 

 absorbs, besides carbonic-acid gas, some of the acetic acid which has escaped condensa- 

 tion, and some of the phenols, which are, however, but imperfectly absorbed. The 

 enormous amount of lime needed in purifying wood-gas is a great inconvenience in 

 this manufacture. 



The following table shows by percentages the composition of gas from different 

 kinds of wood after purification : 



We see from this, that wood-gas, like coal-gas, is a mixture of the oxide of carbon, 

 hydrogen, marsh-gas, and the heavy hydro-carbons which, render it illuminating. 

 Among the latter we may mention acetylene, olefient gas, propylene, benzine, toluene, 

 xylene, &c. The production of the heavy carburets takes ])lace csi^ecially at the be- 

 ginning of the distillation, and goes on diminishing very rapidly, so that the gas, toward 

 the end of the i)rocess, has very little illuminating power. According to Liebeg's ex- 

 periments, the light of wood-gas is to that from coal as 6 is to 5. 



The density of wood-gas is quite considerable, and varies between 0.6 and 0.7, which 

 is chiefly due to the large projiortiou of the oxide of carbon which it contains. But 



