Ratio of the Specific Heats for Air and Steam. 227 



partly with the object of testing this point that the present 

 investigation was undertaken. 



If 6 } and 0. 2 be the initial and final temperatures of the 

 gas, and p x and p 2 the initial and final pressures respectively, 

 then according to the well-known relation 



' log(pi/^)-log(^ 2 )' * * ' 

 If then the gas be allowed to expand in such a manner that 

 Pu P2j #i an d 2 can De measured, the ratio (7) of the specific 

 heat at constant pressure to the specific heat at constant 

 volume can be calculated. 



In the case of steam, which could not be considered as a 

 perfect gas at the temperatures at which the present experi- 

 ments were made, the characteristic equation proposed by 

 Callendar (Proc. R. S. Ixvii. 1900) was employed. On this 

 assumption the adiaba tic relation is still given by equation (1). 



Part I. 



2. Experiments with Air. 



The apparatus employed is shown on Plate I. It consisted 

 of a large spherical copper vessel (not shown in the figure) 

 which we will call the " air-vessel, " of about 50 litres 

 capacity, connected to a tube C for admitting the air to be 

 experimented with ; into the " air-vessel " passed a platinum 

 thermometer by means of which the fall of temperature on 

 expansion of the air at a point near the centre of the vessel 

 was measured. Into the neck of the vessel was soldered a 

 side tube of 1*8 cm. diameter. By withdrawing a rubber 

 stopper fitting tightly into this tube the pressure in the 

 vessel was allowed to fall from a value (p^ previously ad- 

 justed to the atmospheric pressure (p 2 ) . By means of the 

 tube D the " air-vessel " was connected to an oil mano- 

 meter M which could be placed in communication with the 

 experimental vessel or cut off from it at will by means of 

 the glass tap E. The usual arrangement for measuring 

 the resistance of the platinum thermometer is also shown in 

 the figure. In connexion with the " air-vessel " was a 

 mercury-gauge N which served as an automatic key for 

 closing the battery-circuit at a definite instant after releasing 

 the pressure of the air. The gauge N was connected by 

 rubber r to a T-piece in the tube 1), through which passed 

 a platinum wire it;, just dipping into the mercury when the 

 pressure inside the apparatus was equal to the pressure of 

 the atmosphere. Dipping into the other arm of the gauge 



Q2 



