Ratio of the Specific Heats for Air and Steam. 233 



The value of y obtained with a platinum-blacked thermo- 

 meter was 1*360 for t = 0*86 second. 



Since the value of y corresponding to r = 0*86 sec. is 1*394 ? 

 the correction to be applied for radiation is 



l- 3 94-l-360 = . 0()24- 



14 



Hence the corrected value of the ratio of the two specific 

 heats of air is 7=1*401. 



Part II. 



7. Experiments with Steam. 



It will be readily understood that in order to determine 

 the ratio of the two specific heats for steam, the use of vessels 

 of the size employed in the experiments with air just described 

 would be exceedingly inconvenient; and indeed the large 

 size of the vessel does not seem to present the same advantage 

 as in experiments made by the method of Clement Desormes. 

 In the latter method the whole of the gas contained in the 

 vessel is being experimented with, and consequently any error 

 due to the heating of the gas close to the walls produces 

 serious errors in the value of 7 obtained ; it is therefore 

 desirable to reduce the surface of the vessel compared 

 with its volume. In the present method, however, it is 

 merely with the variation of temperature at the point 

 where the thermometer is situated with which we are con- 

 cerned 5 and any heating of gas near the walls of the vessel 

 is unimportant. It therefore seemed likely that results of 

 equal accuracy to those obtained with a large vessel might 

 be obtained with a far smaller one. 



To test this point experiments were made with air con- 

 tained in smaller vessels, and the following apparatus was 

 finally constructed for the steam experiments. 



A cylindrical copper vessel with coned ends of about 

 9*3 litres capacity (PI. I. fig. 3) was constructed having a 

 wide tap A, by opening which the steam could be allowed to 

 expand adiabatically. 



A tube D, provided with a tap through which the vessel 

 could be filled with steam, passed through the lower ex- 

 tremity. On either side of the vessel tubes (B and C) were 

 attached; through B a platinum thermometer was inserted : 

 C communicated through a. tap and a fine tube E with a 

 glass tube which was connected to the oil manometer and 

 automatic key. The whole was inclosed in a copper jacket 

 filled with steam maintained at an excess pressure oi' about 



