30 
Proceedings of the Royal Society of Edinburgh. [Sess. 
Table I. — Prehensility at -190° C. 
Substance. 
N itrogen. 
Hydrogen. 
Plumstone charcoal . . . . . . . 
. 10-2 X 9-0 
10-3 X 1-2 
Birch charcoal ........ 
x8-6 
Cocoanut charcoal ....... 
x4-5 
10-3 X 0-6 
German impregnated charcoal .... 
x3-6 
50 per cent, silica (A) ; 50 per cent, cocoanut charcoal 
X 3’2 
S.S. mixture . 
x3T 
Silica (A) 
X 1-6 
Common wood charcoal ... ... 
X 1-6 
Silica (B) ......... 
xO-7 
Activated anthracite ....... 
X 0*7 
into use ; or again, we may wish to ascertain how far the preliminary 
evacuation ought to go to enable the charcoal to bring the pressure down 
to a desired point. When the prehensility is known such problems admit 
of easy solution. 
The vacuum-producing power of a known weight of adsorbent of a 
known prehensility may be calculated thus : — 
L is the volume, in litres, of the space to be exhausted. 
w is the weight, in grams per litre at N.T.P., of the gas contained in 
the space. 
A is the weight, in grams, of the adsorbent placed in connection with 
the space. 
(T is the prehensility of the absorbent at the temperature to which it 
is cooled. 
The space is first roughly pumped out to a pressure P mms., and the 
final pressure, after the adsorbent has been used, is mms. mercury. Also 
let 0^ be the initial absolute temperature of the space (usually room tem- 
perature), and 0^ its mean absolute temperature when the adsorbent is 
in use. 
When the pressure is P, the weight of gas in the space is X j 
Jjpw 273 
when the pressure isy>, it is 
760 ^ a 
The weight of gas adsorbed per 
gram of substance is therefore 
As p is very small. 
cr 
0359 ™/ 
9 
weight gas adsorbed per gram 
P 
