MM. Jamin and Richard on the Cooling of Gases, 243 



serving as a vehicle for the heat;, and being in unstable equili- 

 brium between the gain and the loss. 



But its mode of conductivity is quite special. Let us in 

 thought divide the gaseous mass into tw^o concentric equal parts 

 by an impermeable partition placed between the thermometer 

 and the enclosure. We can imagine the interior mass alone 

 being heated 2t, taking an excess of pressure 2h. If we open 

 the partition, the pressure and the temperature will fall r and h 

 in this masS; but will rise as much in the part exterior to the 

 partition ; both will then have the same pressure and the same 

 heating. This reasoning may be repeated by multiplying the 

 partitions; and on passing afterwards to continuity, we find that 

 the heat is transmitted from the thermometer as far as the en- 

 closing boundary with and by the transmission of the pressures, 

 and that the temperature is the same at every point. But a 

 thermometer placed at one point yAW not give this temperature ; 

 for it will receive and absorb the heat radiated through the gas, 

 of which it will receive so much less as it is nearer one side of 

 the vessel. 



It will be remarked that, the transmission of the pressure 

 being instantaneous, it will be the same with the propagation of 

 the heat from the centre to the exterior — and that if different 

 gases becom.e heated or cooled more or less rapidly, this can only 

 depend on the greater or less rapidity with which they take heat 

 from a heated solid surface or give it up to the wall of the vessel 

 which contains them. In short, gases have an instantaneous 

 internal conductivity, and place themselves in equilibrium of 

 temperature and pressure. These conclusions, however, sup- 

 pose that they are diathermanous. 



The part played by gases being so well defined, it will be con- 

 ceived how their cooling-power can be deduced. Let us replace 

 Dulong^s therm.ometer by a wire heated by means of an electric 

 current to an excess of temperature t. It will lose during each 



unit of time a quantity of heat equal to —Pc-y- and give, it up 



to the gas. This will take excesses of temperature and pressure 



dO and hj and transfer to its envelope the quantity of heat pc -7-. 



When the stationary condition is attained, this gain and this 

 loss will be equal, and we shall have 



P ^ _ ^^^ n\ 



dx dx ^ ' 



On the other hand, the heat given up by the gas to the en- 

 closure is proportional to the surface of contact 5, to a factor 

 which will be special for each gas, and to a function of H and 



R2 



