HEATING-POWER. 



SPECIFIC-HEAT EFFECTS continued. 



109 



The third or economic method for determining lntil ing- 

 power resemhles that in which wood is burned in the ordinary 

 manner. 



Equal volumes of different woods are burned in a stove or 

 furnace, the heat of which is shown by means of a thermo- 

 meter, or by a steam-engine the steam from which is measured 

 by a manometer. This method shows that in order to burn 

 wood in our ordinal 1 } so much air is required to keep 



up the burning, that half the heating-effect is lost by the 

 draught up the chimney. 



The scale given above, shows that specific weight is the 

 chief factor in heating-power, so that the. heaviest wood in a 

 tree, or of different species of trees, gives the greatest heat. 

 Only in the case of woods of nearly the same speciiic weights 

 do other factors intervene. The measures detailed on page 58 

 to produce heavy wood also ensure good heating-power, as 

 specific weight and heating-power correspond. 



As lignin is richer in carbon than cellulose, every factor 

 that increases the lignin in wood also increases its heating- 

 power (cf. p. 79). 



The water contained in wood, as in sapwood, may attain 

 50 per cent, of the weight of the wood ; then 45 per cent, 

 of the heating-power is used to drive off the water as steam 

 and only a small percentage remains for heating the stove. 



As regards floated and rafted wood the same considerations 

 apply that have ueen already described (p. 102). They are 

 especially liable to be attacked by fungi, chiefly species of 

 Corticium. A special investigation of the fungi that attack 

 floated wood is desirable. 



Nothing reduces the heating-power of wood more than the 



