SOLID FUELS. 87 



Brix made a number of experiments in using wood for 

 heating, and found that dry pine gave the best results 5 

 pounds per pound of fuel. Elm gave 4.6 pounds; birch, 

 4.6; oak, 4.56; ash, 4.63; and beech, 4.47. 



Wood should be dry as possible, as otherwise it has to 

 furnish heat to vaporize, not only the water formed from its 

 hydrogen, but also that already existing as moisture. We 

 have seen that this loss with coal is considerable, it is still 

 greater with wood. Suppose the wood to be ordinary air-dried, 

 containing 20 per cent of water. If this wood, when per- 

 fectly dry, could evaporate 5 pounds of water, it now has 

 only of that power, or power to evaporate 4 pounds; but it 

 already carries \ of its weight of water, which must be vapor- 

 ized. Hence the available power is 4 pounds less -^ pound = 

 3| pounds, or 76 per cent of its dry value. Hence the 

 economy of using only dried, and even artificially dried, wood. 



