98 On two New Pressure- Gauges for the Highest Vacua. 



ciable departure from Boyle's law ; for if there were, there 

 could not be a constant relation between density as found by 

 the M'Leod gauge and by the other two; but as to departure 

 from Boyle ; s law due to other causes than surface condensa- 

 tion the agreement tells nothing, as it simply means an agree- 

 ment in three measurements of density. But it is of some 

 importance to learn that surface condensation produces no 

 appreciable effect, even by way of an apparent departure from 

 Boyle's law in rare gases (see " Boyle's Law at very Low 

 Pressures ") ; of course it is not implied that there is no sur- 

 face condensation, for, on the contrary, its nature and amount 

 have been calculated in "Boyle's Law at very Low Pressures," 

 but in ordinary vessels the mass condensed is a small constant 

 fraction (for a given gas) of the total mass in the vessel, and 

 produces no effect on Boyle's law to the degree of accuracy 

 of measurement hitherto attained. 



The trace of water-vapour which was the probable cause of 

 the abnormalities discussed in connexion with hydrogen is also 

 the probable cause of the failure of Crookes's M'Leod gauge 

 at pressures near and below 1/10 6 atmo both with hydrogen 

 and air. As at the lowest pressure in air the M'Leod gauge 

 gave a measurement of the pressure only 1/26 of that of the 

 viscometer gauge, it follows that 25/26 of the gas in the 

 highest vacuum would appear to have been H 2 0, which, when 

 the gas is compressed into the volume-tube of the M'Leod 

 gauge, simply recornbines with the glass again ; if this is so, 

 then the pressures calculated from the log. dec. and from the 

 deflecting force both for air and hydrogen are inaccurate, as 

 they ought to be calculated with the values of the parameters 

 for H 2 instead of those for air and H 2 . In fact, if at the 

 highest vacua in both air and H 2 water-vapour is the main 

 constituent, then I, the log. dec, ought to have the same value 

 both for air and H 2 at the same erroneous values of the 

 pressure given by the M'Leod gauge. The following little 

 table furnishes a comparison of the log. decs.: — 



apparent p . . . 



1-3 



1-0 



•37 . 



•16 



10 4 / for air.., 



, 175 



161 



126 



115 



10 4 ZforH 2 ... 



164 



157 



130 



118 



This shows that our supposition as to water's forming the 

 chief ingredient of Crookes's highest vacua is probably true, 

 and therefore the pressures which we previously obtained 

 from the viscometer and transpiration gauges at the highest 

 vacua are erroneous, being calculated for air and H 2 instead 

 of for H 2 0, but obviously the amount of their error cannot 



