634 LECTURE LI. 



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 pf 

 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 sub- 

 jected to the friction of various substances as it revolved ; and he found that 

 the soft filaments of the cotton excited much mpre heat, than any other of tlie 

 substances employed. 



The chemical productioii of heat is of greater practical importance^fhan its 

 mechanical excitation ; but by what means chemical changes operate in excit- 

 ing 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 aeriform 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 uncon- 

 cerned. It appears on the whole, that however heat may be excited, the corpus- 

 cular poAvers of cohesion and repulsion are always disturbed and called into 

 action, their equilibrium being destroyed and again restored, whether by me- 

 chanical or by chemical means. A wax candle, ^ 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 experi- 

 ments of Mr. Lavoisier and Mr. Laplace, the combustion of ten grains of 

 phosphorus requires the consumption of 15 grains of oxygen, the combustion 

 of ten grains of charcoal 2C, and of hydrogen gas 56; and by the heat pro- 

 duced during the combustion of a pound of phosphorus, 100 pounds of ice 

 may be melted, during that of a pound of charcoal 96i, of hydrogen gas 

 ^95^ of wax 133, and of olive oil 149; and during the deflagration of a 

 pound of nitre with about one sixth part of its weight of charcoal, twelve 

 pounds of ice may be melted. 



The manner in which heat, when excited or extricated by any of these 

 means, passes from one body to another, requires to be very particularly exa- 



