THE CARBOHYDRATES PRODUCED IN CROPS 131 



3000 acres ordinary timber. It would also serve the purpose 

 of any moderate-sized works dealing with about 150 to 600 

 tons of waste wood per annum, according to whether the 

 machine was w^orked continuously or not. With small 

 plants it is quite impractical to attempt to conserve the 

 acetone and methyl alcohol. For the purpose of obtaining 

 charcoal, however, small forms are more economical. The 

 old-fashioned method of burning charcoal in heaps (see 

 Bibliography) produces a charcoal with a high percentage of 

 ash, which for many industrial purposes is extremely objec- 

 tionable. Distillation in retorts produces a purer charcoal, 

 but for the purpose of obtaining a charcoal with little ash 

 larger pieces of wood only should be carbonized. For the 

 preparation of high-class charcoal for industrial purposes a 

 small plant is, therefore, more manageable, as it can be used 

 to produce charcoal of any particular kind. For annealing 

 or case-hardening steel a charcoal powder containing a high 

 percentage of volatile matter is preferred. Where this 

 is the case, the temperature of distillation must be kept 

 below that stated above. Where a dense charcoal is required, 

 long protracted heating is necessary. For average conditions 

 the period of distillation will occupy three or four hours for 

 each foot in the diameter of the retort. With small laboratory 

 size retorts distillation can take place in under half an hour, 

 but in large retorts running up to eight feet in diameter two 

 days will be found necessary. Bigger retorts than this are 

 not practicable. Small pieces of wood distil distinctly more 

 quickly than large pieces. When coniferous wood is distilled, 

 a valuable product is turpentine. A ton of hard wood on 

 distillation gives about eighteen gallons of water with little 

 acid in the first fraction, which is hardly worth saving, 

 and thirt}^ gallons of strong pyro-ligneous acid in the second 

 fraction. The economy in treatment by this fractionation 

 compensates for some of the disadvantages of an intermittent 

 machine. 



Charcoal from coconut shells has a high absorptive 

 power for gases or vapours. 



(d) Gum and Mucilage. — The name " gum " is a general 



