M. V. Regnault on the Expansion of Gases. 139 



city than that of sound, the effects of the elasticity of the air are 

 annulled, and the compression produced by the moving body 

 has not time to reach the adjacent layers before they in their 

 turn are compressed by the body. In consequence of this inertia 

 the air becomes compressed as it would be in a pneumatic sy- 

 ringe. A great part of the heat which is the result of this com- 

 pression passes into the moving body, the temperature of which 

 it raises. Moreover the moving body is not affected by the ex- 

 pansion of the air producing cold; for this expansion does not 

 take place till after it has passed. Thus, in my opinion, the 

 moving body always, proceeding with the same velocity, collects 

 by the compression of the air the heat which it disengages, and 

 is not affected by the cooling produced by the subsequent expan- 

 sion of the layers of air which it has just traversed. 



It is evident moreover that the compression of the air will be 

 more energetic the greater the velocity of the moving body ; the 

 temperature of the moving body will rise then successively until 

 it is equal to that assumed by a layer of air which instantane- 

 ously undergoes the same compression in the pneumatic syringe. 

 Thus is well explained the very high temperature which a me- 

 teorite assumes when traversing the air With a far greater velocity 

 than that of the propagation of sound. 



A heating of the same kind, but less, would be produced in 

 a moving body which traversed the air with a lower velocity 

 than that of sound. In this case also the moving body would 

 be more influenced by the heat disengaged by compression than 

 it would be by the absorption of heat by expansion. The two 

 effects would virtually compensate each other when the velocity 

 was very small. 



In my opinion there is no heat disengaged by the friction of 

 two bodies, except when the molecules of at least one of them 

 are not quite free — that is to say, when they are under the influ- 

 ence of some force of aggregation. From our observations this 

 absolute liberty would only be met with in the imponderablefluids, 

 such as the sether which transmits the luminous vibrations. It is 

 not perfect in our gases; and from this fact alone the motion of a 

 gas against a solid must disengage a certain quantity of heat, which 

 results solely from the transformation into heat of the loss of vis 

 viva undergone by the molecules in overcoming their internal 

 resistances. My experiments prove that this quantity of heat 

 is so small for atmospheric air that it escapes our means of ob- 

 servation. 



Liquids have always more or less viscosity, which proves that 

 their particles have not perfect mobility. The passage of a liquid 

 through a tube must therefore disengage by friction an appre- 

 ciable quantity of heat; and this quantity must vary with differ- 



