The Economic Production of Artificial Heat. 399 



so great, that it is burned or allowed to rot after having been 

 cut down. Hence, the writer has seen in Canada localities sur- 

 rounded by forests, and yet supplied with coal, brought from 

 the United States by lake and rail, almost as economically, 

 as with wood. In old countries, from the great value of land, 

 and often from the abundance of coal, the supply of wood is 

 very limited. But it might be grown for constructive and en- 

 gineering purposes, very abundantly in Great Britain and 

 still more so in Ireland, if the vast extent of mountain 

 and other unproductive land were devoted to it. Not only 

 would a large amount of profit be thus secured to the owners, 

 but the appearance of the country would be improved, and (on 

 account of the changes produced in the atmosphere by growing 

 plants) its healthfulness would be augmented. The value of 

 wood, where nothing else can be profitably cultivated, and its 

 effect in beautifying wild scenery, are not as much remembered 

 as they should be. On the continent of Europe, where it is 

 grown in large quantities, the most romantic views derive their 

 interest from picturesque and extensive forests. 



Many and valuable experiments have been made on the 

 heating powers of the various kinds of fuel ; but they can be 

 looked upon merely as approximations, since not only the 

 various species, but the various specimens of the same species, 

 differ greatly from each other. The heat-giving power of fuel 

 depends on the nature and proportions of its constituents. 

 Though it is intimately connected with the quantity of oxygen 

 which enters into combination during combustion, it would be 

 incorrect to assert that a pound of that supporter will cause the 

 same amount of heat to be evolved, whatever the substance with 

 which it unites ; for something depends also on the nature of 

 the combustible, since the quantities of heat obtained from dif- 

 ferent subtances, with an equal absorption of oxygen, are not 

 the same. Thus, hydrogen, during combustion, gives out four 

 times as much heat as an equal weight of cai-bon, though it 

 unites with only three times as much oxygen ; and fourteen 

 times as much heat as an equal weight of sulphur, though it 

 unites with only eight times as much oxygen. Wood, and 

 other kinds of fuel, owe their heating capabilities to carbon, 

 or to carbon and hydrogen. But the highest duty of a pound 

 of carbon may be considered as about fifteen pounds of water 

 evaporated, after having been raised to a temperature of 212°; 

 and of a pound of hydrogen, as about sixty-two and a half pounds. 

 These quantities suppose that all the heat is carried off by the 

 waste steam ; but, if the heat of the latter is utilized in any way, 

 the theoretical amount of duty is increased. Any oxygen present 

 in fuel diminishes the effect derived from the hydrogen, since 

 the heat obtained from it is lessened by any portion of it being 



