458 Analyses of Books. [Dec. 



a nature, that they were not easily susceptible of a better ar- 

 rangement than our author has given them : but there was a 

 very obvious method of destroying the want of connexion which 

 always looks ill in a philosophical work. The book might have 

 been divided into as many chapters as there were subjects dis- 

 cussed. This would have rendered the whole much more per- 

 spicuous and entertaining ; and, I am persuaded, would have 

 made the book much more generally read than it will be. In its 

 present state it is best adapted for those who are already pretty 

 well acquainted with the doctrine of heat. Had it been divided 

 into chapters, it would have been easily understood, so perspicu- 

 ously is it written, by every person in the least curious about 

 such subjects. In my analysis of the book I shall take the liberty 

 (for the sake of my readers) of supplying Mr. Leslie's omission, 

 and dividing it into its various heads. 



I. Sketch of the Facts respecting Heat. — Mr. Leslie, with the 

 o-reater number of philosophers of the present day, thinks that 

 the phenomena of heat are best explained by considering it to be 

 a fluid of a very peculiar nature, either the same as that which 

 produces light, or a modification of it. He gives us a short 

 sketch of the facts known respecting the conducting power of 

 different bodies, and their different capacities for heat. He 

 adopts 135° for the latent heat of water. This is the number 

 which Mr. Cavendish informs us he obtained by his own experi- 

 ments. From the well-known accuracy of Mr. Cavendish, there 

 is every reason to believe that his number is entitled to be pre- 

 ferred to 140°, the number which Dr. Black deduced from his 

 experiments. At the same time, it were to be wished that Mr. 

 Leslie had informed us of the reason which induced him to adopt 

 135° in preference of 140°. Did he repeat the experiment 

 himself, and obtain 135°? Or did he rely solely upon the 

 authority of Mr. Cavendish ? 



Mr. Leslie accounts for the heat evolved during combustion 

 by the change of capacity which the substances concerned in 

 combustion undergo. Thus when charcoal is burnt, the oxygen 

 of the air is changed into carbonic acid; and the superiority of 

 the capacity of oxygen gas above that of carbonic acid is the 

 reason of the enormous quantity of heat evolved during the 

 combustion. This is the mode of explanation which was first 

 given by Dr. Black, and afterwards adopted by Dr. Irvine, and 

 Lavoisier, and Laplace. 1 endeavoured about ten years ago to 

 show that it was quite inadequate to account for the phenomena; 

 and 1 still consider my arguments as perfectly conclusive. The 

 new experiments of Berard and Delaroche on the specific heat 

 of the gases put the inadequacy of the explanation adopted by 

 Mr. Leslie, and the philosophers just named, in a very striking 

 point of view. The specific heat of oxygen gas is 0*23$ 1, while 



