PUEL AND CHAECOAL 91 



of wood-pulp in 1899 exceeded 20,000 tons, valued at nearly two 

 millions sterling. These amounts have undoubtedly at the present 

 time been largely exceeded. 



Fuel. — ^The heat-producing value of wood as fuel varies greatly, 

 owing to the differing capacity that woods have for retaining 

 moisture. Thus, while green wood may contain 50 per cent, of 

 moisture, ordinary stack-wood may contain only 25 per cent., and 

 kiln-dry wood only 2 per cent. With 26 lbs. of water, 100 lbs. of 

 fire-wood will contain about 1 lb. of incombustible ash and 74 lbs. 

 of the dry substance of wood. This last consists of 37 lbs. of carbon, 

 32 lbs. of oxygen and 4-4 lbs. of hydrogen ; and in burning the whole 

 of the oxygen combines with 4 lbs. of hydrogen to form water, so 

 that only the 37 lbs. of carbon and 0-4 lb. of hydrogen — ^'.e., about 

 haK the weight of the dry substance of the wood — are available for 

 heat-production. Every pound of water combined in the wood 

 requires about 600 units of heat to evaporate it, the unit being the 

 amount of heat necessary to raise 1 lb. of water 1° C. ; so that 

 100 lbs. of stack-wood (25 per cent, moisture) only furnishes about 

 255,000 units, whilst if Idln-dry (2 per cent.) it would yield 350,000, 

 The advantage of seasoning for firewood is, therefore, obvious. 

 The resinous woods of the conifers produce most flame and are 

 most useful accordingly in starting a fire ; but the denser hard 

 woods produce from 25 to 30 per cent, more heat. 



Charcoal and distillation of wood.— When wood is heated to 

 200° E. without access of air, it remains unaltered, at 220° it becomes 

 brown, and at 270° to 300° it suffers decomposition, torrefied wood 

 or red charcoal being formed. At 350° it is resolved into volatile 

 products and true or black charcoal. If the temperature is raised 

 gradually, so that 600° E. is not reached for several hours, the pro- 

 cess is called dry distillation. The first product of distillation is 

 almost entirely water ; but at 500° pyroligneous (crude acetic) acid, 

 or wood-vinegar, wood-spirit and uncondensable gases pass off, 

 charcoal and some tar remaining. In the primitive method of the 

 charcoal-burner, or meiler, in which billets of wood are stacked 

 horizontally or inclined round a central chimney opening, most of 

 the volatile products are lost ; but for charcoal this process is still 

 largely employed on the Continent. If the fire is steady and 

 regular, the slower the process the better the yield. Eor gun- 

 powder-charcoal, however, and acetic acid, iron or brick ovens are 

 mostly employed. The best gunpowder- charcoal is produced from 

 light woods, such as Willow, Buckthorn, or " Dogwood " {Rhdmnus 

 Frdngula), and Alder. Charcoal is darker, heavier, a better con- 

 ductor of heat and electricity, less easily ignited, and gives out 

 greater heat in burning, the higher the temperature at which it has 



