208 Mr. W. Sutherland on the Spontaneous 



of molecules of 2 per unit volume in the hot chamber, over 

 its value in the cold chamber must be 3/2 the corresponding 

 difference in the values of N 2 /B, that is to say, the difference 

 of the partial pressure of the 2 in the hot and the cold must be 

 3/2 the difference of the partial pressure of the 3 in cold and 

 hot; so that the total excess of pressure in the hot chamber over 

 that in the cold is equal to half the difference between the 

 partial pressures u a and d h \ but we have just seen that with the 

 temperatures constant these are constant over a large range of 

 pressure from the pressure at which dissociation just begins up 

 to the pressure at which the first discontinuity occurs. Thus, 

 then, as the regions of the bulb on the two sides of the mica 

 plate in Crookes's experiment correspond to our hot and cold 

 chambers, we see theoretically that the deflecting force ought 

 to be constant from a pressure of about *7 millim. or 920/10 6 

 atmo down to about 2a! or *11 millim. or 144/10 6 atmo, 

 according to our equations with one value of k and *055 

 millim. as the value of a. Eow in Crookes's data already 

 given we saw that the deflecting force remained practically 

 constant at pressures from 1000/10° atmo down to between 

 300/10 6 and 200/10 6 in his list of apparent pressures, which 

 are really p-\- a ; so that the deflecting force remains constant 

 down to a pressure between 228/10 6 and 128/10 6 in close 

 enough agreement with the 144/10 6 given above. When 

 the degree of rarefaction is reached at which dissociation is 

 just impossible in the hotter chamber, then the deflecting 

 force will become that of pure 3 , and its laws will be those 

 of a single gas as given in " Thermal Transpiration and 

 Radiometer Motion " ; in the region of pressure between 

 that at which dissociation becomes impossible in the hot 

 chamber and that at which it becomes impossible in the cool 

 chamber, there will be a fall of deflecting force from that due 

 to pure 3 to the constant value which rules up to the pressure 

 of discontinuity. 



As to the comparatively small variations of 2a\ /B D which 

 we found in our study of the log. dec, their smallness would 

 be explained by the supposition that the viscosity of 3 is 

 very nearly the same as that of 2 , for then the limiting con- 

 stant value L of the log. dec. of 2 would also be approxi- 

 mately a correct value for all the different values of L 

 appropriate to all the different mixtures of 3 and 2 , which 

 must have been experimented on at pressures between 

 1000/1 6 and 144/10 6 atmo. But it must be mentioned 

 that Crookes drew attention to a slight irregularity in the 

 values of the log. dec. of oxygen, for after maintaining a 

 value nearly constant at about "1120 near a pressure of 

 20 millim., it rose to -1124 at 7*5 millim., after which it fell 



