242 MM. Jamin and Richard on the Cooling of Gases. 



pends only on the gas — very small for carbonic acid^ greater for 

 air^ and very considerable for hydrogen. It is assumed that v^ 

 measures the effect of the gas. 



In reasoning thus^ Dulong and Petit make a pure hypothesis, 

 and probably commit an error. It is always possible to repre- 

 sent the total velocity V by the sum v-\-v' -, but it is not proved 

 that V expresses the radiation as it exists in the gas, and v^ the 

 cooling due to the gas itself. On the contrary, it is probable 

 that the radiation is less than?;, since the gas is imperfectly 

 diathermanous, as Tyndall has proved, and consequently that 

 the effect attributable to the gas must be augmented by so much. 



Besides, Dulong and Petit appear to have taken no account 

 of the kind of action exerted by the gas ; at least they do not 

 attempt to explain it. They present that action as a fact ; they 

 assume that the gas remains at the temperature of the enclosure, 

 that the presence of the thermometer changes neither its tem- 

 perature nor its pressure. The apparatus they employ would 

 not permit the ascertaining of this change, if it took place. Thus 

 they overlooked the true conditions of the problem. 



In truth, the gas is warmed, and its pressure increases. Let 

 us take a glass balloon immersed in water, furnished with a sen- 

 sitive manometer, and traversed along one of its diameters by a 

 fine and resisting platinum wire. As soon as it is heated by an 

 electric current, we shall see the manometer rise progressively 

 and the temperature of the gas increase. One of us, in a pre- 

 vious study, even ascertained that the heating was much greater 

 in proportion as the pressure and volume were less. 



This observation explains to us, in the first place, the pertur- 

 bations discovered by MM. de la Provostaye and Desains in the 

 ease of very small enclosures and feeble pressures. The gas 

 being then very much heated, its temperature can no longer 

 be confounded with 6 (that of the enclosure) and the excess t 

 measured by the difi'erence between the degree of the thermometer 

 and 6. The real excess is smaller; the factor t^ must be dimi- 

 nished_, either by replacing t by its true value and diminishing 

 dj or by attributing to d decreasing values variable with the 

 pressure H. 



But this fact has a still greater importance in that it clearly 

 reveals the part played by the gas during the cooling. It is 

 heated by contact with the thermometer, and transmits the heat 

 to the exterior covering, which absorbs it. At the first instant 

 it receives more than it gives up; and the manometer rises pro- 

 gressively W'ith a decreasing velocity, then remains stationary 

 when the heat taken up by the thermometer is equal to that 

 which it yields to the enclosure. The gas, Iherefore^, acts as a 

 conducting niass, being heated on one side, cooled on the other, 



