214 WELLS'S NATURAL PHILOSOPHY. 



lustrate the effect of friction in producing heat: — A borer was made to re- 

 volve in a cylinder of brass, partially bored, thirty-two times in a minute. 

 The cylinder was inclosed in a box containing 18 pounds of water, the tem- 

 perature of which was at first G0° but rose in an hour to 107°; and in 

 two hours and a half the water boiled. 



Is air necessary ^^^ ^°®^ "°'' ^^P^^^ ^0 be necessary to the production of 

 for the produc- heat by the friction of solid bodies • suice heat is produced 



frictiou?''''' ^ ^y ^''''-■ti"" ^vi^l"^ a vacuum. 



To whatever extent the operation may be carried, a body 

 D3ver ceases to give out heat by friction, and this fact is considered as a 

 strong argument in favor of the theory that heat is not a substance, but 

 merely a property of matter. 



It was formerly supposed that solids alone could develop heat by friction, 

 but recent experiments have proved, beyond a doubt, that heat is also gener- 

 ated by the friction of fluids. 



The heat excited by friction is not m proportion to the hardness or elas- 

 ticity of the bodies employed ; on the contrary, a piece of brass rubbed with 

 a piece of cedar- wood produced more heat than when rubbed with another 

 piece of metal ; and the heat was still greater when two pieces of wood were 

 employed. 



The reduction of matter into a smaller compass by an exter- 

 tratronTof^'the °^^ °^ mechanical force, is generally attended with an evolu- 

 production of tion of heat. To such an act of compression we apply the 

 heat by con- • . i j.- 



densation ? term condensation. 



Heat may be evolved from air by condensation. This may 

 be shown by placing a piece of tinder in a tube, and suddenly compressing the 

 air contained in it by means of a piston. The air being thus condensed, parts 

 with its latent heat in sufficient quantity to set fire to the tinder at the bot- 

 tom of the tube. Another famiUar experiment of the evolution of heat by 

 condensation, is the rendering of iron hot by striking it with a hammer. The 

 particles of the iron being compressed by the hammer, can no longer contain 

 so much heat in a latent state as they did before : some of it therefore be- 

 comes sensible, and increases the temperature of the metal, and the striking 

 may be continued to such an extent as to render the iron red-hot. 



"When a match is drawn over sand-paper or other rough substance, cer- 

 tain phosphoric particles are rubbed off, and being compressed between the 

 match and the paper, their heat is raised sufficiently high to ignite them and 

 fire the match. If the match be drawn over a smooth surface, the compres- 

 sion must be increased, for the temperature of the whole phosphoric mass must 

 be raised in order to cause ignition. 



The fulminating substance of a percussion-cap explodes when struck by a 

 hammer, because the blow occasions a compression of the particles, by which 

 a sufficient amount of latent heat is liberated to produce ignition. 



What is meant ^^^- Most livinc^ animals possess the property 

 by vital heat? ^£ maintaining in their system an equable tern- 



