SOLID FUELS. 85 



Wood contains, besides, small quantities of mineral matter and 

 hygroscopic water varying from 15 to 30 per cent, according 

 to dryness. Air-dried, it contains about 15 per cent of water, 

 which it gives up easily on exposure to a heat of 100 C. 



The composition of wood may be represented by the 



following: 



Carbon. Hydrogen. Oxygen. Ash. Water. 



Wood dried at 100 49.5 6.0 43.5 i.o o.o 



" li in the air 29.6 4.8 34.8 0.8 29.0 



Regarding wood from its ultimate composition, we may 

 consider it as a hydrate of carbon, that is, as carbori united to 

 water, the proportion of hydrogen and oxygen being nearly 

 the same as in water. But regarded from its proximate com- 

 position, it is entirely different. What has been said of soft 

 coal can be repeated for wood ; that, those having a similar 

 ultimate composition behave differently in distillation in a 

 closed retort and produce very different proportions of carbon 

 (as charcoal) ; hydrocarbons, liquid or gaseous ; acid products, 

 resin, and tar. It was supposed that the heat of combustion 

 differed also, and this has been verified by experiments. 



Berthelot and Vielle determined the heat of combustion of 

 cellulose, and found 680 calories for the molecular weight of 

 wood, or about 4200 calories per kilogram. 



Hard wood gives less heat than soft wood. According to 

 Gottlieb's experiments, pine-wood has a heat value of 5000 

 calories, while oak gave only 4620 calories. Mahler's exper- 

 iments confirm a difference in favor of pine, but in less pro- 

 portion. 



Two determinations made by Mahler are (cinders and water 



deducted) : 



Fir. Oak. 



Carbon 51.08 50.43 



Hydrogen..., 6.12 5.88 



Oxygen with trace of nitrogen. ... 42.90 43-69 



100.00 100.00 



Heat of combustion 4828 4689 



