Pressure- Gauges for the Highest Vacua. 97 



inner surface of each of the two chambers connected by the 

 annular space to a uniform temperature. 



We will now briefly discuss the performances of the two 

 gauges with C0 2 , CO, and ~N 2 by means of Crookes's measure- 

 ments of log. dec. and deflecting force, for using the latter of 

 which the appropriate values of c r , A", and B //x are given in 

 " Thermal Transpiration and Radiometer Motion ;" for using 

 Crookes's values of the log. dec. we have the following: — 



C0 o . CO. N 2 . 



10 4 L 822 968 970 



2aX p /D ... 15 17 16*6 



co 2 . CO. 



10 4 log. dec 424 347 325 298 474 448 305 



def. for 16 11 10 8 14 13 7 



p by gauge 15 10 9 7 "6 13 12 65 



p from log. dec. ... 14-4 9'7 8-6 7'4 149 134 6-8 



p from def. for. ... 14*5 9'3 8-3 66 131 119 57 



N,. 



10* log. dec 420 351 318 257 207 178 



def. for 17 14 13 9 3 1 



p by gauge 13 9-6 83 5-8 33 2-8 



jp from log. dec. ... 115 8"3 74 54 3'6 2"9 



p from def. for. ... 10-5 8'1 73 4-9 1-5 "5 



In the case of N 2 the pressure from the log. dec. and that 

 from deflecting force agree down to about 5/10 6 atmo, although 

 systematically less than those by the M'Leod gauge and 

 Boyle's law; but at the two lowest pressures a great difference 

 appears, which is too large for the same explanation as applied 

 to the change of ratio with air and H 2 ; in comparison with 

 the deflecting force in air that in N 2 dies away in a very 

 sudden manner — for example, while at 3/10 6 atmo the de- 

 flecting force in air is about 10, in N 2 it is only 1, a 

 very remarkable difference, calling for further experimental 

 inquiry. 



Crookes's measurements for oxygen, being of special im- 

 portance, will be discussed separately in my next paper. The 

 fairly good agreement which we have found between the in- 

 dications of the M'Leod gauge a ^ low densities and those of 

 the viscometer gauge and the transpiration gauge furnish 

 an indirect proof that there can be no surface condensation 

 which suffices with the gases discussed to produce any appro- 



