484 



LECTURE LI. 



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 persuaded that what we call heat is, in its intimate nature, rather 

 a mechanical 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 permanent : 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 

 peculiar sensation which bears its name, and of which the diminution pro- 

 duces 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 positive cold. Both these ideas are simple, and each of them 

 might be derived either from an increase or from a diminution of a positive 

 quality : but 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 experience 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 produced in those bodies to which heat is 

 attached in an increased quantity, and the contraction of those from which 

 it is subtracted. 



The simplest modes of exciting heat appear to be the compression of 

 elastic fluids, and the collision or friction of solid bodies ; although a more 

 usual and a more powerful source of heat is found in various chemical 

 combinations and decompositions, which are produced by the peculiar 

 elective attractions of different substances for each other, or from the influ- 

 ence of the solar rays, which are probably emitted in consequence of the 

 chemical processes that continually take place at the surface of the sun. 



The effects of the condensation and rarefaction of elastic fluids are 

 shewn by the condenser and the air pump ; when an exhaustion is made 

 with rapidity, the thermometer, placed in the receiver of the air pump, 

 usually sinks a degree or two ; and when the air is readmitted abruptly 

 into a partial vacuum, the sudden condensation of the rarefied air raises 

 the mercury : and a similar elevation of temperature is produced by the 

 operation of the condenser. Much of this heat is soon dissipated, but by 



