SCIENCE OF COMMON THINGS. 227 



Amount of water in green and dry wood. Weight of wood. 



easily, and, from the admixture of atmospheric air in its 

 pores and crevices, burns out rapidly, producing a com- 

 paratively temporary, though often strong heat. 



1513 In recently cut wood, what proportion of its weight is water ? 



From one-fifth to one-half. 



1514 After wood has been dried in the air for ten or twelve months, 

 how much water does it usually contain ? 



From twenty to twenty-Jive per cent. 



1515 Why do we call some woods hard, and others soft f 



This distinction is grounded upon the facility with 

 which they are worked, and upon their power of pro- 

 ducing heat. Hard woods, as the oak, beech, w T afnut, 

 elm, and alder, contain in the same bulk more solid 

 fibre, and their vessels are narrower and more closely 

 packed than those of the softer kinds, such as pine, 

 larch, chestnut, etc. 



1516 How many pounds avoirdupois are there in a cord of dry, hard 

 wood? 



From forty-four hundred pounds in a cord of dry 

 hickory, to twenty-six hundred in a cord of dry, soft 

 maple. 



151*7 What is the most valuable wood for fuel? , 



The varieties of hickory ; after that, in order, the 

 oak, the apple-tree, the white-ash, the dog-wood, and the 

 leech. 



1518 What woods give out the least heat in burning f 



The white-pine, white-birch, and poplar. 



1519 Why is it unprofitable to burn green wood or wet coal f 



It is a well-known law of heat that the evaporation 

 of liquids, or their conversion into steam, consumes or 

 renders latent a great amount of caloric. When green 

 wood or wet coal are added to the fire, they abstract 

 from it by degrees a sufficient amount of heat to con- 

 vert their, own" sap or moisture into steam before they 

 are capable of being burnt. As long as any consider- 

 able part of this fluid remains unevaporated, the com- 

 bustion goes on slowly, the fire is dull, and the heat 

 feeble. 



