WOOD.PKESERVING 1155 



through sieves, and then packed in barrels ready for use. It had the appearance of 

 pigconu' dung. 



In France the paste obtained by pounding the woad-leaves was taken to a room 

 with a sloping pavement, open at one end, laid in a heap at the higher end of the 

 room, and allowed to ferment for a period of twenty or thirty days. The mass 

 swelled up and often showed cracks or fissures, which were always carefully closed as 

 goon as they appeared, whilst a black juice exuded and ran away in gutters constructed 

 for the purpose. When the fermented heap had become moderately dry, it was ground 

 again and formed into cakes, called in French coques, which were then fully dried, and 

 in this state brought to market. In France and Italy a second fermentation was not 

 generally thought essential, but when performed it was conducted exactly in the manner 

 just described. 



At the present day woad is nowhere employed alone for the purpose of dyeing 

 blue, since it is found more economical to use indigo, and the cultivation of the plant 

 has therefore declined considerably, and has even become nearly extinct in districts 

 whore it was formerly carried on extensively. By woollen dyers, however, it is still 

 used, but only as a means of exciting fermentation, and thus reducing the indigo blue 

 in their vats ; indeed, the woad employed by them contains little or no blue colouring- 

 matter. See INDIGO. 



Numerous attempts have been made to extract the blue colouring-matter from 

 woad, in the same way that indigo is extracted from the leaves of the Indigofera in 

 the East Indies and other countries. At the commencement of the present century, 

 when the price of indigo on the Continent of Europe was very high, a prize of 

 100,000 francs was even offered by the French Government for the discovery of a 

 method of obtaining from the Isatis tinctoria, or some other native plant, a dyeing 

 material, which, both in regard to price and the beauty and solidity of its colour, should 

 form a perfect substitute for indigo. The experiments which were made in Conse- 

 quence served to prove that it was quite possible to obtain genuine indigo from woad- 

 leaves, but that the process could never bo carried on profitably on account of the 

 very small proportion of colouring-matter contained in the plant. Nine parts of fresh 

 leaves yield only one part of the prepared material or pastel, and the latter does not 

 afford more than 2 per cent, of its weight of indigo. According to Chevreul, the 

 leaves of the Indigo/era anil, even when grown in the neighbourhood of Paris, con- 

 tain 30 times as much indigo-blue as those of the Isatis tinctoria, and, when cultivated 

 in tropical countries, the amount is probably still higher. The comparatively high 

 price of land and labour would probably itself prove a sufficient obstacle to the suc- 

 cessful manufacture of indigo in most European countries, even if the yield were equal 

 to what it is in the tropics. 



In 1808 Chevreul published the results of his analysis of woad and pastel. It has 

 more recently been made the subject of chemical investigation, for the purpose of 

 ascertaining the state in which indigo-blue exists in plants and other organisms. See 

 INDIGO. E. S. 



WOOD-PRESERVING. The preservation of wood from decay depends upon 

 the combination of the vegetable albumen with some metallic salt or some powerful 

 antiseptic agent. Bethell's invention, which was much employed, consists in im- 

 pregnating wood throughout with oil of tar and other bituminous matters containing 

 creosote, and also with pyroliguite of iron, which holds more creosote in solution 

 than any other watery menstruum. 



The wood was put in a close iron tank, like a high-pressure steam-boiler, which 

 was closed and filled with the tar oil or pyrolignite. The air being exhausted by 

 air-pumps, afterwards more oil or pyrolignite was forced in by hydrostatic pumps, 

 until a pressure equal to from 100 to 150 Ibs. to the inch was obtained. This pres- 

 sure was kept up by the frequent working of the pumps during six or seven hours, 

 whereby the wood became thoroughly saturated with the tar oil, or the pyrolignite of 

 iron, and weighed from 8 to 12 pounds per cube foot heavier than before. 



In a large tank 20 loads of timber per day could be prepared. The atmospheric 

 action on wood thus prepared renders it tougher, and infinitely stronger. A post 

 made of beech, or even of Scotch fir, is rendered more durable, and as strong as onn 

 made of the best oak ; the bituminous mixture with which all its pores are filled 

 acting as a cement to bind the fibres together in a close tough mass ; and the more 

 porous the wood is, the more durable and tough it becomes, as it imbibes a greater 

 quantity of the bituminous oil, which is proved by its increased weight. The materials 

 which are injected preserve iron and^metals from corrosion ; and an iron bolt driven 

 into wood so saturated remains perfectly sound and free from rust. It also resists the 

 attack of insects. 



The effect produced is that of perfectly coagulating the albumen in the sap, thus 

 preventing its putrefaction. For woocl that will be much exposed to the weather, and 



4 B 2 



