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



ent liquids. I have several times made experiments in this direc- 

 tion, but they are not yet complete enough for publication. 



In solids, lastly, especially in those which are very hard, the 

 molecules have very little mobility ; the friction of these bodies 

 against each other must, then, produce considerable disengage- 

 ment of heat, even when there is no disaggregation. In all cases 

 the disengaged heat arises from the transformation of a percep- 

 tible external motion into a molecular vibratory motion about the 

 position of equilibrium^ which is only manifested by its calorific 

 effects ; so that the disengagement of heat by friction always cor- 

 responds to a loss of vis viva. 



Mathematical theory supposes that bodies possess perfect elas- 

 ticity ; this is implicitly assumed in establishing the primordial 

 equations. It is supposed, moreover, that the motion cannot 

 change its nature nor be communicated to other media. Lastly, 

 no account is tiaken of the materiality of bodies ; for no allowance 

 is made for the modifications which gravity introduces into mo- 

 lecular motions. In the case of the sether its gravity need not be 

 taken into account ; but the modification is recognized which the 

 elasticity of the sether undergoes in transparent material sub- 

 stances, because it is necessary to admit this in order to explain 

 refraction. 



Hence the laws deduced from the mathematical theory of elas- 

 ticity should only be capable of absolute verification in the case 

 of media which possess perfect elasticity ; and we only observe 

 this in the case of the aether which transmits luminous vibra- 

 tions. Experiment shows us that the velocity of propagation in 

 the sether is excessive as compared with what we observe in mate- 

 rial bodies. May we not conclude thence that in a medium of 

 absolutely perfect elasticity the velocity of propagation would be 

 infinite? According to this, the sether alone approaches far 

 towards this perfection ; but there still remains to it some mate- 

 riality, which leaves it under the influence of material bodies, and 

 in virtue of which it offers an appreciable resistance to the motion 

 of the stars which traverse it. 



However this may be, I think that the degree of elastic perfec- 

 tion of a body may be appreciated by the velocity with which it 

 transmits a disturbauce. The velocity of propagation is greater 

 in solids than in liquids, and greater in liquids than in gases. 

 Hence gases should deviate from the mathematical laws of elas- 

 ticity more than solids and than liquids, which in fact is what 

 experiment proves. 



Determination of the quantities of heat absorbed by a gas in motion 

 which undergoes sudden or successive variations of density, by ob- 

 serving the temperatures it possesses in different parts of its course. 

 If it were possible to determine simultaneously the tempera- 



