38 ON THE HEAT EVOLVED 



made perfectly dry. and afterwards exposed in a room in which 

 no fire was made during a period of twelve months, the average 

 absorption by weight, for this period, was found to be 10 per 

 cent, in forty six different woods, and 8 per cent, in the driest 

 states of the atmosphere, and an unexpected coincidence was 

 found to exist in the absorption by weight of forty six pieces 

 of charcoal made from the same kinds of wood, and similarly 

 exposed, the latter being also 8 per cent. 



The quantity of moisture absorbed by the woods individually, 

 was not found to diminish with their increase in density ; while 

 it was found that the green woods, in drying, uniformly lost less 

 in weight in proportion to their greater density. Hickory wood 

 taken green, and made absolutely dry, experienced a diminu- 

 tion in its weight of 37|- per cent., white oak, 41 per cent, 

 and soft maple, 48 per cent. ; a cord of the latter will therefore 

 weigh nearly twice as much when green as when dry. 



If we assume the mean quantity of moisture in the woods, 

 when green, as 42 per cent., the great disadvantage of attempt- 

 ing to burn wood in this state must be obvious, as in every 100 

 pounds of this compound of wood and water, 42 pounds of 

 aqueous matter must be expelled from the wood, and as the 

 capacity of water for absorbing heat is nearly as 4 to 1, when 

 compared with air, and probably greater during its conversion 

 into vapour, which must be effected before it can escape, the 

 loss of heat must consequently be very great. 



The necessity of speaking thus theoretically on this point, 

 is regretted ; but, it will be apparent, that this question of loss 

 cannot be solved by my apparatus, as the vapour would be con- 

 densed in the pipe of the stove, and the heat would thereby be 

 imparted to the room, which, under ordinary circumstances, 

 escapes into the chimney. 



The average weight of moisture in different woods which 

 have been weather seasoned from eight to twelve months, will 

 not be found to vary materially from 25 per cent, of their 

 weight; every economist, therefore, will see the propriety of 

 keeping his wood under cover in all cases where this is practi- 

 cable. 



