30 SOURCES OF HEAT. 



a metal in an acid, by the combination of the two 

 electricities, positive and negative, by the absorption 

 of light, and even by the rubbing of two solid bodies 

 together with a certain degree of rapidity, heat may 

 be produced. 



By a number of causes, in appearance entirely dis- 

 tinct, we can thus produce one and the same effect. 

 In combustion and in the production of galvanic elec- 

 tricity we have a change of condition in material par- 

 ticles ; when heat is produced by the absorption of 

 light or by friction, we have the conversion of one 

 kind of motion into another, which affects our senses 

 differently. In all such cases we have a something 

 given, which merely takes another form ; in all we have 

 a force and its effect. By means of the fire which 

 heats the boiler of a steam-engine we can produce 

 every kind of motion, and by a certain amount of mo- 

 tion we can produce fire. 



When we rub a piece of sugar briskly on an iron 

 grater, it undergoes, at the surfaces of contact, the 

 same change as if exposed to heat ; and two pieces 

 of ice, when rubbed together, melt at the point of 

 contact. 



Let us remember, that the most distinguished author- 

 ities in physics consider the phenomena of heat as 

 phenomena of motion, because the very conception of 

 the creation of matter, even though imponderable, is 

 absolutely irreconcilable with its production by me- 

 chanical causes, such as friction or motion. 



But, admitting all the influence which electric or 



