ox THE MEASUKEHEXT OF TEM I’EK ATUlo:. 
24. The Pressure Coefficients of Hydrorjen and Helium at Lower Initicd' 
Pressures. 
Though the accuracy of the determlnatiou.s of the pressure coefficients at Inwer 
pressures is reduced in proportion to the initial pressure, the results we have obtained 
indicate that the })ressure coefficient for the two gases is practically independent of 
the pressure. The value of the coefficient found for liydrogen at an initial pressTire 
of 500 millims. is 0'0036()27, and for helium 0'003GG25 and 0‘003GG31 ; if the 
coefficient attained a limiting value corresponding to 0'003GGO at zero pressure, the 
value at 500 millims. should he 0’003GG12. 
The determination of the pressure coefficient for hydrogen at an initial })ressure of 
350 millims. is practically valueless; only one set of measurements was made, and 
we found that our metliod was not sufficiently accurate to investigate the coefficient 
at such a low pressure. 
25. <P-nerai Conrlnsions. 
(h-iArPUrs has determined the pressure coefficients of nitrogen at dlffierent pressures 
with the followino- results - 
Initial pre.s.sure. 
C'oefticienl. 
inillinis. 
1000 
0-00.367445 i 
79:1-5 
0-0036718 
5.30-8 
0-0036683 1 
On plotting tlie values of the coefficient against tlie initial pre.ssure, tlie value of' 
the coefficient at zero initial pressure is found on linear extra])olation to lie between 
0-003GG2 and 0-003GG3. 
As Daniel Bektiielot (‘Conpjtes Kendus,’ 1898) has pointed out, the specific 
volumes of the common gases at zero pressure, calculated from their densities under 
normal conditions, and the variation of “ p.v.” with ])ressure, are the same. Theli- 
densities uiider the same conditions are proportional to their chemical atomic weights. 
If this is the case, they should, at zero pressure, Ijehave as perfect gases, and tlie 
temperature scale on a thermometer filled witli any gas at verv low ])ressure sliould 
he coincident with the absolute scale of temperature. 
