AMOUNT ADSORBED 
26 
Proceedings of the Royal Society of Edinburgh. [Sess. 
III. — Prehensility : a Factor of Gaseous Adsorption. 
By Professor Henry Briggs, D.Sc., Ph.L). 
(MS. received November 7, 1921. Read November 7, 1921.) 
The term ‘‘ retentivity ” never appears to have received exact definition ; 
nor does it convey the meaning required, for example, when considering 
the function of charcoal in producing high vacua, where the conception 
of gripping or seizing hold of the gas molecules is more pertinent 
than that of their retention. For these reasons, and also because such 
methods as have been devised of measuring “ retentivity ” rest on arbi- 
trary adjustments and are difficult to correlate, it is here suggested that 
the term “ prehensility ” be adopted in place of the older and vaguer 
expression, and that a definitive meaning be attached to it. 
The deg;ree of activation of a char- 
coal has frequently been judged from 
its capacity, at a given temperature, 
for a gas or vapour at atmospheric 
pressure. Such a measurement of 
capacity has, however, no relation to 
the charcoal’s power of adsorbing 
gas or vapour at low pressure or low 
partial pressure. As I have shown 
elsewhere,* a colloidal silica may be 
prepared which, on nitrogen at liquid 
® PRESSURE air temperature and atmospheric pres- 
sure, has 166 per cent, of the capacity 
of the best cocoanut charcoal ; yet its vacuum-producing power is greatly 
inferior to that of the charcoal. Plotting weights of gas adsorbed per gram 
of adsorbent as ordinates and the corresponding pressures of the gas as 
abscissae, a curve resembling A, fig. 1 (diagrammatic only), is obtained for 
the charcoal and one similar to B for the silica. Clearly, what matters in 
evacuation or in dealing with an adsorbent’s ability to abstract small pro- 
portions of vapour from air is the slope of the isotherm at the origin ; this 
slope, I suggest, should be called the 'prehensility of the substance for the par- 
ticular gas at the particular temperature. If this definition be accepted. 
dp Jo 
(f) 
* Proc. Roy. Soc., A, 100, 1921, p. 88. 
