COMBUSTIBILITY AND HBATINO POWEB. 29 



(4).. Smallness of the pieces of wood used. — The smaller the 

 pieces are, the larger is the surface exposed to a free draught of 

 air, and the greater the quantity of heat' evolved. But there must 

 he a limit to the smallness of the pieces, for sawdust burns with 

 little heat, as it does without flame. 



(5). Presence of oils and resin. — This circumstance requires no 

 explanation. 



(6). Soundness. — Unsoundness necessarily implies some loss 

 of the original quantity of carbon and hydrogen, the only two 

 combustible elements in the composition of wood. As all trees 

 begin to get more or less unsound at the centre after a certain 

 age, trees intended for the supply of firewood ought not to be kept 

 beyond middle age. 



The popular belief that floating diminishes the heating power 

 of wood is totally unfounded. What actually happens is that 

 when floated wood is taken out of the water, the pieces are piled 

 up pell-mell into large heaps, inside which they undergo a certain 

 amount of decomposition. If the wood is dried at once, no loss of 

 heating power will result from the floating. 



Numerous attempts have been made to ascertain the relative 

 heating capabilities of the various woods. In some physical methods 

 have been employed, in others chemical methods. 



The most common physical method is to ascertain what quantity 

 of water at 0° C. is evaporated by one pound of the given wood 

 at a given temperature of the air and under a given pressure. 

 A simpler method is to find out what quantity of ice at 0° C. is 

 converted into water of a temperature of 0° C. by one pound of the 

 wood. A third method, having another purpose, is to burn sepa- 

 rately the same quantity of the several woods in one and the same 

 fireplace, and note the diflference between the temperatures of the 

 air of the room at the beginning and end of each burning. The 

 doors and windows of the room should of course remain closed 

 during the experiment. 



Chemical methods consist in ascertaining the quantity of carbon 

 and hydrogen present in a given weight of the wood. This is 

 done by burning the wood in a closed retort, either with a direct 

 supjjly of oxygen gas or with a known weight of some metallic 

 oxide. In the former case we know at once the quantity of 

 oxygen used up, in the other we weigh the balance of the oxide 

 and thus ascertain the quantity of oxide reduced, and therefore of 

 oxygen given up to the burning wood. In either case we are en- 

 abled to calculate the quantity of carbon and hydrogen burnt. 

 The physical methods give results of very little practical value, 



