Dr Fyfe en the Evaporative Power of Coal. 257 



Much, however, yet remains to be clone ; and now, when owing 

 to the rapid increase of steam-machinery the demand for fuel 

 is so greatly increased, it becomes the duty of all who can add 

 to the sum of our information, to make public the result of 

 any experiments they have made on this important subject. 



Very different opinions have been, and are still, entertained 

 regarding the source of heat during combustion, and of the 

 power of different inflammables for evolving heat. The re- 

 cent experiments of Despretz on this subject are perhaps the 

 most important, as tending to the deduction of a law by 

 which, if correct, we shall be enabled to calculate with accu- 

 racy the amount of heat evolved by different combustibles. 

 From his numerous experiments, Despretz has drawn the con- 

 clusion, that the heat disengaged during combustion is in pro- 

 portion to the quantity of oxygen with which the combustible 

 unites. Thus applying this rule to hydrogen, carbon, alcohol, 

 and ether, and taking the quantity of water raised from 32° to 

 212° as the means of measuring the comparative amount of 

 heat disengaged, he found by experiment that the quantity of 

 water brought to the boiling point by the union of 



1 lb. of oxygen with hydrogen, was . 29£ lb. 



Do. with carbon, was . . 29 



Do. with ether, was . . 28^ 



Do. with alcohol, w»s . • . 28 



results so very nearly agreeing witb each other, that, keeping 

 in view the unavoidable sources of error in all experiments of 

 tbis kind, we may consider the quantity the same in all. The 

 average of the above trials is 28f. Hence, when one pound of 

 oxygen enters into union with any inflammable, heat is 

 evolved, which, according to Despretz, is sufficient to raise 

 28f lb. of water from the freezing to the boiling point. It may 

 be stated in round numbers as 29 lb. We have thus, then, a 

 method of procuring a standard for the amount of heat disen- 

 gaged during combustion. Thus 1 of carbon unites with 2.66 

 of oxygen, and 2.66 x 20 = 77.14, so that, by this process of 

 calculation, lib. of carbon ought to raise 77.141b. of water 

 from 32° to 212°. This is rather below what is stated by Desp- 

 retz. He has fixed it at 78.15. 



Different statements have been given of the quantity of ca- 



