86 Mr. W. Sutherland on two New 



pressure in a vacuum becomes less delicate when I becomes 

 nearly equal to L, for then the value of p is liable to be greatly 

 affected by only a small error in I. Thus, for example, a 

 diminution of I in the first case from "0988 to *098G would 

 reduce the calculated pressure to 1000 ; thus the first two or 

 three values of the calculated pressures belong to a range 

 where this particular torsion radiometer is not sensitive as a 

 pressure-gauge. Of course we are at present assuming that 

 the pressures as given by Crookes's M'Leod gauge are 

 correct ; it must be remembered that what the indications of 

 the M'Leod gauge really amount to is a sort of a measure of 

 the density of the gas, with an inference as to its pressure by 

 means of Boyle's law, and I have already pointed out that 

 the measurement deduced from slipping is strictly only a 

 measurement of density with a pressure inferred by means 

 of Boyle's law ; the fact, therefore, that the pressures as found 

 by the log. dec. and by the M'Leod gauge in the last table are 

 on the whole so consistent furnishes no proof of the validity of 

 Boyle's law at low pressures, but this, however, has been 

 fairly established on other grounds in " Boyle's Law at very 

 Low Pressures," and therefore the consistency of the two sets of 

 pressures is a proof of the relative correctness of the pressures 

 obtained by Crookes with his M'Leod gauge. 



The great advantage about the new viscosity-meter gauge 

 is that its sensitiveness tends to increase with diminution of 

 the pressure which it has to measure. Let us therefore 

 proceed to compare the pressures of air as found by Crookes's 

 M'Leod gauge at still lower pressures, and as calculated by 

 (34) from his measurements of the log. dec. 



p by gauge 72 5'9 41 34 2 6 19 1-3 1-0 



p from log. dec. ... 79 67 5'0 44 3'6 30 2-5 2-2 

 10* log. dec 372 337 281 256 225 198 175 161 



p by gauge '55 -46 -22 14 -06 -02 



. p from log. dec. ... 1-82 1*65 1-33 125 '95 -53 



10 4 log. dec 144 135 118 114 97 72 



Here we have increasing divergence between the two values 

 of the pressure, till at a pressure estimated as "02/10 6 atmo by 

 the M'Leod gauge and Boyle's law the log. dec. in the 

 viscosity-meter gives a pressure 26 times as great. Now it is 

 rather important that we should have some test as to which of 

 the above series of pressures is the more reliable, because 

 although from what we know of the tenacious cling of H 2 

 to glass there is reason to expect the M'Leod gauge to fail at 

 some point, still we cannot without further support attribute 



