26 HABIT AND INTELLICtENCE. [chap. 



as might be expected, is alike true, whatever be the process 

 by which the transformation of the energy of motion into 

 the heat is effected : whether it is by collision, as when a 

 projectile flashes tire against the target ; ^ or by friction, 

 which may perhaps be regarded as consisting of an infinite 

 number of molecular collisions ; or, as in one of the expe- 

 riments by which Professor Joule ascertained this law, by 

 transforming energy of motion into electricity, and this 

 into heat ; or by the comjjression of air. 



It is scarcely necessary to say, that when all the energy 

 of motion of a moving body is transformed into heat, the 

 body ceases to move. 

 Heat is This fact, that whatever quantity of energy of motion 



motion.^ disappears is represented by an equivalent quantity of heat, 

 makes it highly probable that heat is really motion, the 

 motion of the molecules of the heated substance ; so that 

 the transformation of energy of motion into heat is really 

 the transfer of the energy of motion from the mass of the 

 body that ceases to move to the molecules of the bodies 

 that become heated. In a word, heat is molecular motion. 

 I do not say that the facts already mentioned are sufficient 

 to prove this : I admit that they only suggest it. But 

 this theory is confirmed by all that we have learned of 

 the action of gases under pressure ; and the dynamical 

 theory of heat is now as well established as the undulatory 

 theory of light. 

 All matter I may here remark, though it is a digression, that the 

 elasti/°* ^' ^^^^ ^^ ^^'^ apparently lost energy of motion being trans- 

 formed into heat, or molecular motion, proves the very 

 important and previously unknown truth, that all matter 

 is perfectly elastic. If two balls of a higlily elastic sub- 

 stance, such as ivory, come into collision, they rebound 

 with great force, and very little of their energy of motion 

 disappears. But if two balls of lead, or of any other almost 

 inelastic substance, come into collision, they rebound with 



1 " Mr. Fairbaim iufoiins me that in the experiments at Shoebury- 

 ness it is a common thing to see a flash of light, even in broad day, when 

 the ball strikes the target." (Tyndall on Heat as a Mode of Motion, 

 p. 437.) 



