NATURE OF HEAT. $7 



temperature of several pounds of cold water to the boiling point. 

 Sir H. Davy succeeded in melting two pieces of ice in the 

 vacuum of an air pump,, by making them rub against each other, 

 while the temperature of the air pump itself and the surrounding 

 atmosphere was below 32. M. Haldot observed that when the 

 surface of the rubber was rough, only half as much heat ap- 

 peared as when the rubber was smooth. When the pressure of 

 the rubber was quadrupled, the proportion of heat evolved was 

 increased seven fold. When the rubbing apparatus was sur- 

 rounded by bad conductors of heat, or by non-conductors of 

 electricity, the quantity of heat evolved was diminished.* No 

 heat whatever is produced by the friction of fluids upon each 

 other, or upon solids ; nor by the friction of gases upon liquids 

 or solids. 



One other point only connected with the nature of heat re- 

 mains, to which there is at present occasion to allude the 

 existence of a repulsive property in heat. Such a repulsive 

 power in heated bodies is inferred to exist from the appearance 

 of extreme mobility which many fine powders assume, such as 

 precipitated silica, on being heated nearly to redness. Mr. Forbes 

 also attributes to such a repulsion the vibrations which take 

 place between metals unequally heated, and the production of 

 tones, to which allusion has already been made. But this re- 

 pulsive power was rendered conspicuous, and even measurable, 

 by Mr. Powell, in the case of glass lenses, of very slight con- 

 vexity, pressed together. On the application of heat, a separa- 

 tion of the glasses, through extremely small but finite spaces, 

 was indicated by a change in the tints which appear between the 

 lenses, and which depend upon the thickness of the included 

 plate of air. This repulsion between heated surfaces appears to 

 be promoted by whatever tends to the more rapid communi- 

 cation of heat.f 



* Nicholson's Journal, XXVI, 30. 

 t Phil. Trans. 1834, p. 485. 



