106 MODERN SCIENCE READER 



ture, and when the wood comes to be burnt its calorific value 

 is found to vary slightly owing to these factors, and also to 

 the amount of moisture which it contains ; as upon the con- 

 stituents of the sap will largely depend the amount of ash 

 which is formed, and upon the moisture the amount of 

 heat which will be rendered latent in the conversion of the 

 water into steam. Moreover, in the formation of the cellu- 

 lose oxygen equivalent in quantity to that which was orig- 

 inally in combination with the hydrogen will have been 

 again taken into combination in the formation of the plant's 

 structure, with the result that air-dried wood, when tested 

 for its calorific value, is but a poor fuel. 



The moisture present in a sample of wood will vary 

 enormously with the time of year at which the tree has 

 been cut down, and also with the nature of the tree, so that 

 while as little as eighteen per cent, of moisture has been 

 found in one kind of wood, it may exceed fifty per cent, in 

 another, while, under the most favorable conditions, air 

 drying will only reduce the moisture in wood to form 

 eighteen to twenty per cent. It may, therefore, be roughly 

 stated that at the best, wood will only contain eighty per 

 cent, of combustible matter, while the large amount of heat 

 absorbed in heating and evaporating the water present is 

 a serious drawback to it as a fuel. 



The combined oxygen also present in the cellulose, as has 

 been before indicated, seriously detracts from its value, 

 and where wood is the only fuel that can be employed, 

 and great local heat is required, a fuel of practically double 

 the value of wood can be obtained by its conversion into 

 charcoal before use. Under the influence of destructive 

 distillation the contained moisture and combined oxygen 

 are driv r n forth as water vapor ; and although four fifths 

 of the weight of the wood is lost in the liquid and gaseous 

 products escaping, yet the twenty per cent, of carbon that 

 remains on burning is free from the drawback of having 

 the intensity of the heat of combustion lowered by the 

 rendering latent of heat, which, in the case of wood, was 



