125 
1920-21.] The Adsorption of Gas under Pressure. 
pressures, slightly increasing again. If the pressure is carried high enough, 
this effect is found to be common to all the pressure-volume relations we 
have examined, provided the relationship is of the non-linear type. At 
15° C. and with the gases concerned the possibility of a change of phase 
at the higher pressures is nil, and another explanation has to be sought 
for the effect. 
The internal gaseous volume of an adsorbent granule is made up partly 
of openings of molar dimensions and partly of much larger canals or 
capillaries, some of which may be visible under the microscope. With 
these gases of low critical temperature, even under pressures of about 
100 atmospheres, the adsorbed film cannot occupy the whole of the internal 
gaseous volume, some of which must therefore be occupied by gas, not 
adsorbed, but approximately obeying Boyle’s law. Thus it appears neces- 
sary to modify equation (1) so as to include a term to cover the effect 
of compression upon unadsorbed gas existing in the capillaries. Let v be 
the volume (expressed at N.P. and 15° C.) taken up by a gross litre of the 
granules, the volume compressed in the interstitial spaces not being 
included. Let v 1 be the volume taken up by true adsorption upon the 
solid surfaces, and v 2 be the volume contained in the capillaries under 
simple compression. 
Then from equation (1) 
log^-A^V, (2) 
and 
v = + v 2 , 
but, by Boyle’s law, v 2 = kp % , therefore 
v = v l + kj) . . . . . (3) 
Equation (3) thus expresses the extent of the modification of Williams’ 
rule needed to take into account the existence, in the granules, of gas 
under simple compression.* 
The curve for nitrogen in silica (B, fig. 3) was found to agree remark- 
ably well with the equations 
log* (p) = ~ 092 “ '01552), .... (4) 
v = v 1 + ‘10p ...... (5) 
For cocoanut charcoal with nitrogen the equations are 
log, (^) = 2-43 -'045^ (6) 
v = v 1 + -10p ...... (7) 
* Williams lias made use of a similar correction to allow for the volume occupied by 
the adsorbed layer, ibid., p. 306. 
