•2 



CHEMICAL CONSTITUTION OF VEGETABLES. 



From the experiments of Clement, it appears that the heating 

 power of charcoal is equal to 7050 units. Dry wood containing, as 

 we have seen, 52 per cent, of charcoal, its heating power has been 

 deduced theoretically, as equal to 3666. Mr. Marcus Bull in Amer- 

 ica, made a series of experiments to determine the relative quantities 

 of heat given out by different kinds of wood, from which M. Peclet 

 has been led to conclude that the same weight of dry wood of every 

 kind has the same heating power, and that this for a kilogramme, 

 or 2.2 lbs. avoird. of wood dried by artificial means, is equal to 3500 

 units, while the same quantity of the same wood having been cut 

 and seasoned during from ten to twelve months, and containing from 

 20 to 25 percent, of water, is no higher than about 260 units. 



By way of comparison, I shall here add the heating power of the 

 several combustibles in general use, in contrast with that of wood : 



1 kilogrm. or 2.2 lbs. avoird. of wood-charcoal produces 7226 units of heat. 



" coal 6000 " 



" " peat 3005 " 



" " peat charcoal 6400 " 



Although the same quantities of wood, brought to the same degree 

 of dryness, appear to have the same absolute calorific power, all are 

 not alike adapted to the same purposes. Hard woods burn slowly, 

 and give out less heat in a certain time than the less compact kinds 

 of wood. This is the reason why fir is preferred to oak in furnaces 

 where the object is to obtain the most intense heats. It were for- 

 eign to our object to enter upon any consideration of the various 

 qualities, or of the adaptation to particular uses, of different species 

 of limber. I may, however, add a table of the ordinary dimensions 

 of well-grown trees of different kinda, such as are cojuixionly found 

 iu these countries . 



