631 



LECTURE LL 



ON THE SOURCES AND EFFECTS OF HEAT. 



It may appear doubtful to some whether the subject of heat belongs most 

 properly to mechanical or to chemical philosophy. Its influence in chemistry 

 is unquestionable and indispensable; but its mechanical effects are no less 

 remarkable: it could not therefore with propriety be omitted either in a 

 course; of chemical or of physical lectures, especially by those who are per- 

 suaded that what we call heat is, in its intimate nature, rather a mecha- 

 nical affection of matter than a peculiar substance. We shall first inquire 

 into the nature of the principal sources of heat, and next into the mode of 

 its communication, and its most common effects, whether temporary or per- 

 manent: the measures of heat, and the most probable opinions respecting its 

 nature, will afterwards be separately considered. 



Heat is an influence capable of affecting our nerves in general with the 

 pecvdiar sensation which bears its name, and of which the diminution produces 

 the sensation denominated cold. Any considerable increase of heat gives us 

 the idea of positive warmth or hotness, and its diminution excites the idea of 

 posiilve cold. Both these ideas are simple, and each of them might be de- 

 rived either from an increase or from a diminution of a positive quality: bdt 

 there are many reasons for supposing heat to be the positive quality, and cold 

 the diminution or absence of that quality; although we have no more expe- 

 rience of the total absence of heat, than of its greatest possible accumulation, 

 which might be called the total absence of cold. Our organs furnish us, in 

 some cases, with very delicate tests of any increase or diminution of heat; 

 but it is more usually recognised by the enlargement of bulk, generally pro- 

 duced in those bodies to which heat is attached in an increased quantity, and 

 the contraction of those from which it is subtracted. 



