12 RATIO OF SPECIFIC HEATS. 



is evidenced, for instance, by results which show all resistance changes in the 

 bolometer to l)e independent of the time of efflux, within time limits as broad as 2 

 to 8 seconds, in the case of air. True, in the be.st of the earlier experiments, the 

 heat influx from conduction from the walls of the vessel is of serious moment only 

 for the better conducting gases, since the receivers use<l were all of large capacity. 

 RontgeiL, however, Ofienly acknowledges tliat the value of « for hydrogen found in 

 his experiments must be considerably below its real value. We therefore refer to 

 it as conclusive evidence in favor of our method, that, at variance with the results 

 of all earlier investigators, our nietliod actually gives us a larger «-value for 

 hydrogen than for air. 



Neither can the heat whicli enters by nietallic conduction along the electrical 

 terminals of the bolometer adjustment have produced any serious rise of tem[)era- 

 ture — certainly not in that part of the gas immediately around the strip. The thin 

 film of pure platinum by which the measurements are viitually made, is free from 

 silver and (piite distant from the walls of the receiver. It i.s moreover, placed belou^ 

 the levels of the terminals, so that line conduction heat only and not convection 

 heat can reach it. For this reason the bolometer temperature remains constant even 

 for several seconds after the completed expansion of air. 



Regarding the heat imparted by the bolometer to the gas, the following infer- 

 ences may be drawn. W^e have intimated that the measuring current was always 

 reduced to so small a value as not appreciably to change the temperature of the bo- 

 lometer strip. Hence we may abstract from the Joide heat set free within the 

 film altogetiier. Similarly the heat which the bolometer strip gives up to the ex- 

 panding gas is negligibl}- small ; the theiinal capacity of the strip, in view of the 

 dimensions stated, is only about .000 007 gram calories. This is about equivalent 

 to the thermal ca[tacity of -^ cm* of air. Similarly the heat removed from the 

 silver-covered j)arts of the bolometer is without moment, for these cool very ra2)idly. 



Thus it happens that at least those regions of the gas whicli immediately sur- 

 round the etched part of the platinum strip may be regarded as screened from all 

 heat conduction. They therefore expand rpiite adiabatically. 



We have now to consider the question in what degree the temperature 1^ of 

 the gas after ex[)ansion, coincides with the actually measured minimum tempera- 

 ture of the bolometer strip. Since the temperature of the bolometer is constant for 

 several seconds during the observation of 7^, it follows that the bolometer teni[)era- 

 ture can oidy differ fi'om that of the gas if there is a pernianent How of heat into 

 the strip. In sucli a case a stationary distribution of tem[)erature is conceivable, in 

 which the bolometer woidd impart heat to the gas at the same rate in which it 

 receives it. We can l>nl .-icknowledge that there must be accession of heat in the 



