lb 13.] Leslie on Air, Heat, and Moisture. 459 



that of carbonic acid is 0-2210: so that the oxygen has a specific 

 heat -rTo^th parts higher than the carbonic acid. Let us suppose 

 a pound of charcoal consumed. From Lavoisier's experiments 

 it appears that the heat evolved is sufficient to melt 96'^ lbs. of 

 ice. Now this (supposing Mr. Cavendish's estimate correct) is 

 equal to 130271 degrees of heat. The oxygen consumed 

 amounts to 28 lbs. nearly : so that each pound of oxygen, when 

 changed into carbonic acid, must have given off 3428 degrees. 

 Here a change in the specific heat amounting only to r%%-ff, or 

 not quite ^d of the whole, occasioned the escape of 342S 

 degrees. Such a conclusion can only be adopted if we suppose 

 the absolute quantity of heat in the oxygen gas to amount to 

 147101 degrees. This supposition exceeds the estimate of Dr. 

 Crawford nearly 100 times; and it is more than ten times greater 

 than that adopted by Dalton. No person can believe that oxygen 

 gas contains so much heat. Of course the supposition that the 

 heat evolved during combustion is owing to a change of capacity 

 merely cannot be defended. If heat be a fluid, it must enter 

 into chemical combination with certain bodies, and the decom- 

 position of these combinations must be the cause of the heat 

 evolved during combustion. 



Mr. Leslie's notions of the capacity of the different gases for 

 heat, as stated in this treatise, are much more accurate than 

 those of his predecessors, though they do not agree with the 

 results obtained by Delaroche and Berard. Thus he makes the 

 specific heat of hydrogen gas about ten times greater than 

 common air. Delaroche and Berard make it rather more than 

 12 times greater. 



II. Evaporation. — The facts stated by Mr. Leslie respecting 

 evaporation are curious, and some of them are new. What 

 struck me as the most novel and important of his discoveries on 

 this subject is, that the rate of evaporation, like that of the 

 escape of heat from bodies, depends upon the nature of the 

 surface : or that the escape of water by evaporation, and the 

 escape of heat from the surfaces of bodies, depend upon the 

 same law. Water evaporates fastest from those bodies that allow 

 heat to escape fastest, and slowest from those that allow heat to 

 escape slowest ; and it follows exactly the same rate as the escape 

 of the heat. Hence from the surface of glass, charcoal, or 

 paper, water evaporates much faster than it does from metals. 



It would appear from this, that it is the radiant heat chiefly 

 that occasions evaporation. Probably the heat that escapes by 

 the conducting power of the neighbouring bodies is acted upon 

 by the affinity of these bodies, and on that account cannot com- 

 bine with the moisture, and convert it into steam. 



Another important fact with which Mr. Leslie makes us 

 acquainted is the rate at which the capacity of air to retain 



