ISOTHERMAL AND ADIABATIC CHANGES. 293 



be a series of oscillations, the energy of the oscillations being gradually 

 converted to heat, and the entropy being thereby increased. M. Cazin 

 showed the existence of these oscillations, and was able to turn off the tap 

 at the end of the first outrush. The expansion up to that point might 

 be considered as equivalent to an adiabatic one, for nearly all the work 

 done by the gas has gone out of it, the amount retained through viscosity 

 being so far negligible. M. Cazin's determinations were much nearer 

 those given by the velocity of sound than the earlier results. 



Rontgen used as a pressure indicator a corrugated plate like that in 

 an aneroid barometer, fixed over an opening in the reservoir. Any 

 movement of the plate was communicated to a mirror in which was 

 viewed the reflection of a scale. 



He obtained the following values for y : 



Air ...... 1-4053 



Carbon dioxide . . . 1'3052 



Hydrogen . ... 1-3852 



The value for hydrogen is no doubt too small. The gas so rapidly 

 regains the temperature of the surroundings that the adiabatic change 

 of pressure cannot be observed accurately.* 



3. By direct comparison of the two specific heats. 



If a quantity of heat H is given to a unit mass of gas at constant 

 pressure it raises the temperature by dd where 



If V is the initial volume and dv the increase, a the co-efficient of 

 expansion, 



dv = aVdO, 



If the same quantity H is given to the unit mass with constant volume 

 it raises the temperature by dd' where 



If P is the initial pressure, dp its increase, /3 the co-efficient of pressure 

 increase 



H -7sr 



Equating the two values of H, we get 



Cp<fo = 



P 

 OTpPcto 



* Among later researches we may refer to a paper by Capstick, " On the Ratio of 

 the Specific Heats of the Paraffins and their Monohalogen Derivatives," Phil. Trans. 

 A. 1894, Ft. I., p. 1. 



