632 LECTURE LI. 



The simplest modes of exciting heat appear to be. the compression of elas- 

 tic 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 influence of the solar rays, 

 which are probably emitted in consequence of the chemical processes that 

 continually take place at the surface of the sun. '«- :!.If J' 



The effects of the condensation and rarefaction of elastic fluids are shown 

 by the cendenser and the air pump; Avhcn an exhaustion is made with rapid- 

 ity, 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 va- 

 cuum, the sudden condensation of the rarefied air raises the mercury: and a 

 similar elevation of temperature is produced by the operation of the con- 

 denser. Much of this heat is soon dissipated, but by observing the velocity with 

 which the thermometer rises, Mr. Dalton has estimated that air, compressed 

 to half its dimensions, has its temperature elevated about 50 degrees of Fah- 

 renheit; and some of his experiments indicate, when accurately examined, a 

 still greater change. For the present we may define the sense of the term 

 degree, in Fahrenheit's scale, as corresponding to an expansion of a portion of 

 mercury amounting to one ten thousandth part of its bulk; and a degree of 

 Reaumur originally corresponded to an expansion of a weak spirit of wine, 

 amounting to one thousandth part of its bulk. It may be inferred from the 

 velocity of sound, supposing that the excess of its velocity, above the common 

 calculation, is wholly derived from the heat and cold produced by condensa- 

 tion and expansion, that a condensation amounting to ^rs- ^f fhe bulk of 

 any portion of air will raise its temperature one degree of Fahrenheit. 

 When air is very rapidly compressed in the condenser of an air gun, it is 

 sometimes so much heated as actually to set on fire a small portion of tow, 

 placed near the end of the barrel. 



The production of heat by friction is too well known to require an experi- 

 mental proof; but Count Rumford has taken particular pains to ascertain 

 every circumstance which can be supposed to be concerned in the operation 

 of this cause; and the results of his experiments are so striking, that they de- 



