27 
1921-22.] Prehensility : a Factor of Gaseous Adsorption. 
in which <j is the prehensility, M the weight of gas adsorbed per gram 
of adsorbent, and ]) the pressure, or partial pressure, of the gas surrounding 
the substance. 
A. M. Williams * has shown that the most probable form of the 
adsorption isotherm for low concentrations is, 
log.^ = A„-A,M (2) 
I recently found Williams’s rule to hold with reasonable accuracy for 
gases above their critical temperatures, at pressures up to 100 atmospheres, 
providing that a correction be applied for the gas in the capillaries which 
is not adsorbed and is therefore subject to Boyle’s law.f If M be the 
total weight of gas held by one gram of the adsorbent and that part 
which is adsorbed, a more complete form for the isotherm is : 
log^ = A„-A,M, (3) 
P 
M = ...... (4) 
From (1), (3), and (4) we get: 
= + ^ . . . . . • (5) 
Where adsorption is strong — cocoanut charcoal taking up chlorpicrin 
vapour, for example — k may be neglected. 
It is apparent from (3), (4) that the isotherm is virtually a straight 
line near the origin. This inference, which agrees with experimental 
results, { simplifies the determination of prehensility, as it enables 
measurements to be made with relatively coarse vacua. In the instances 
dealt with below (figs. 2 and 3) the isotherms are straight in every 
case up to 0'5 mm., in most cases up to 1 mm., and in several 
beyond 4 mms. of pressure. 
The experiments here described were carried out on behalf of the 
Oxygen Research Committee (Scientific and Industrial Research Depart- 
ment) for the purpose of gaining specific information on the evacuating 
power of the charcoal used in Dewar vessels intended for holding liquid 
air, and to ascertain if it is feasible to substitute colloidal silica for the 
charcoal in those flasks. The prehensilities of a number of activated 
charcoals, and of two grades of silica, were determined at —190° C., the 
boiling temperature of the liquid “ air ” used. Most of the tests were made 
* Froc. Roy. Soc. Edin., vol. xxxix (1918-19), p. 48; Proc. Roy. Soc., A, 96, 1919, 
p. 287. 
t Proc. Roy. Soc. Edin., vol. xli (1921), p. 129. 
I Dewar, Proc. Roy. Inst., vol. xviii (1906), p. 433. 
