HEATING-POWER. 101 



dense compact woods. Porous woods, therefore, burn more quickly 

 and more completely than dense woods, or, in common parlance, 

 light woods give a quicker fire, and heavy woods a more lasting 

 one. Hence it follows that by burning equal weights of dry 

 porous wood, a more intense heat is produced than ^^dth 

 heavier woods. The heating apparatus of houses is usually of 

 such a construction that, after lighting the fire, a certain interval 

 of time must elapse before a room is thoroughly heated. If 

 the development of heat is then very rapid, much of it passes 

 uselessly up the chimney, as the stove is not capable of storing 

 the heat so rapidly. Thus softwoods waste much of their heat 

 when used for heating rooms. In other cases, such as in bakers' 

 ovens, brick- and lime-kilns, the same amount of preliminary 

 heat is not lost, but a rapid intense heat is required, and for 

 these purposes light woods are most suitable. [Thus, wood 

 from Scotch pine thinnings is in great demand by the Paris 

 bakers.— Tr.] 



The degree of reduction in size to which wood has been 

 subjected has a similar influence to that of its porosity. A piece 

 of wood reduced by a plane into thin shavings is a thousandfold 

 more in contact with the oxygen of the air than the entire piece, 

 so that the burning of the shavings would be much more rapid 

 and more complete than that of the latter, and the intensity of 

 the heat given out mich greater. There is, however, a limit to 

 the advantages of subdivision of wood in this respect, for saw- 

 dust merely smoulders, and scarcely emits any flame, when 

 burned. 



4. Amoiuit of liesiii in wood. 



The importance of resin in the heating-power of conifers is 

 well known. Kesinous wood always evolves more heat than 

 wood poor in resin, as the latter adds so much more carbon to 

 the woody substance. 



Old Scotch pinewood, the roots of Scotch pine, mountain 

 pinewood, the lower part of larch-trees, which often contain 

 concretions of resin, portions of the lower part of the stem of 

 spruce-trees where the bark has been injured and which have 

 become encrusted with resin, and resinous old branches of the 

 spruce are, therefore, remarkable for their great heating-pou'cr. 



