Qualities desirable in Fuel — Charcoal. 145 



from the younger ones, the period ranging from 25 to 40 years. If 

 we wish wood for burning only, the coppice is more profitable than 

 full-grown timber, although, in a given quantity of best fire-wood 

 from each, a cord of the latter would be worth the most. 



584. But, for charcoal, the point of greatest excellence is almost 

 reached in middle age, when the difierence is very small, between its 

 excellence then and at full majority.^ 



585. In 1826, Marcus Bull, of Philadelphia, a gold-beater by 

 profession, a member of the American Philosophical Society, and 

 much interested in scientific studies, first published the results of 

 experiments carefully made by him upon the heating and coaling 

 qualities of many kinds, chiefly of native wood. They possess per- 

 manent value, and may be concisely stated as in table on the fol- 

 lowing page. 



586. It should be remembered that woods diflfer according to the 

 soU, aspect, elevatiou, and other conditions under which they have 

 been grown, and that allowance must be made for these differences 

 in applying the above or any other statements. 



Charcoal. 



587. When wood is heated to about 530° (Fahr.), or above, the 

 volatile parts pass off in the form of watery vapor, acetic acid, tar, 

 etc., with certain uou-condensible gases, and there remains a fixed 

 residue called charcoal. 



588. This substance is black, porous, brittle, and when struck, 

 sonorous. It breaks with a glossy conchodial fracture, and when 

 prepared at a low degree of heat, it retains the form and structure 

 of the wood, but in reduced bulk. It is among the most inde- 

 structible of substances when exposed to the open air, or in the 

 ground, and for this reason billets of charcoal are buried in the soil, 

 to mark the corners of sections and their subdivisions in our public 

 land surveys.' 



^Karsten, cited by Loreniz Jj^ Parade, 5 ed., p. 443. In the oak, the propor- 

 tion was 25.45 to 25.00; in the beech, 25.50 to 25.75; and in the hornbeam, 

 24.90 to 26.10. 



'In visiting the ruins of Herculaneum, in 1881, we noticed the woodwork 

 of doors and windows that had been charred by a lava-stream nearly two 

 thousand years ago, in which the kind of wood could be determined, and 

 even the form into which it had been wrought. 

 10 



