486 LECTURE LI. 



produced in this manner, by the operation of friction, was equal to that of 

 9 wax candles, each three quarters of an inch in diameter, continuing to 

 burn for the same time.* 



A still more rapid increase of temperature may be obtained, where the 

 relative velocity of the bodies is more considerable, or where they strike 

 each other with violence. Thus a soft nail may be so heated, by three or 

 four blows of a hammer, that we may light a match with it ;t and by 

 continuing the operation, it may be made red hot : two pieces of wood may 

 also be set on fire by means of a lathe. When a waggon takes fire, for 

 want of having its wheels properly greased, the friction is probably 

 increased by the tenacity of the hardened tar, which perhaps becomes the 

 more combustible as it dries. 



One of the most remarkable circumstances, attending the production of 

 heat by friction, is the discovery of Professor Pictet, that it is often much 

 more powerfully excited by soft substances than by harder ones. In 

 making some experiments in a vacuum, in order to examine how far the 

 presence of air might be concerned in the effects of friction, he accidentally 

 interposed some cotton between the bulb of his thermometer and the cup, 

 which was subjected to the friction of various substances as it revolved ; 

 and he found that the soft filaments of the cotton excited much more heat, 

 than any other of the substances employed.^ 



The chemical production of heat is of greater practical importance than 

 its mechanical excitation ; but by what means chemical changes operate in 

 exciting heat, we cannot attempt to determine. There is certainly no 

 general law of composition or decomposition that can be applied to all such 

 cases : most commonly heat is produced when oxygen exchanges an aeri- 

 form for a solid state, or enters into a new combination, and still remains 

 elastic ; but in the case of gunpowder, heat is disengaged while an elastic 

 fluid is produced from a solid ; and in some other cases the oxygenous 

 principle is wholly unconcerned. It appears on the whole, that however 

 heat may be excited, the corpuscular powers of cohesion and repulsion are 

 always disturbed and called into action, their equilibrium being destroyed 

 and again restored, whether by mechanical or by chemical means. A wax 

 candle, f of an inch in diameter, loses a grain of its weight in 37 seconds, 

 and consumes about three grains, or 9 cubic inches, of oxygen gas, 

 producing heat enough to raise the temperature of about 15,000 grains of 

 water a single degree. According to the experiments of Mr. Lavoisier and 

 Mr. Laplace, the combustion of ten grains of phosphorus requires the con- 

 sumption of 15 grains of oxygen, the combustion of ten grains of charcoal 

 26, and of hydrogen gas 56 ; and by the heat produced during the combus- 

 tion of a pound of phosphorus, 100 pounds of ice may be melted, during 

 that of a pound of charcoal 96, of hydrogen gas 295, of wax 133, and of 

 olive oil 149 ; and during the deflagration of a pound of nitre with about 



* Ph. Tr. 1798, p. 80. Essays, ii. IX. Nich. Jour. ii. 106. See also Haldot, ibid, 

 xxvi. 30. 



t Mem. d'Arcueil, ii. 441. 



J Essais de Physique, Geneve, 1790. 



