L 92 1-22.] Prehensility : a Factor of Gaseous Adsorption. 33 
Actually is usually so large that the optimum value of n is too high 
to be attainable. Thus, if be 10^, will become a maximum when the 
charcoal is divided between 367 bulbs, and the maximal value of will 
be For practical purposes, then, it is permissible to state that the 
evacuating power of a given mass of absorbent can be greatly increased 
by employing it in stages in the manner indicated, and that the more 
numerous the stages the higher the vacuum obtained. 
Summary. 
Prehensility is defined as the slope at the origin of an adsorption 
isotherm. 
A method of measuring prehensility is described, and results given for 
various adsorbents at liquid air temperature. 
It is shown that from the prehensility the evacuating power of a 
substance may be calculated. In evacuating any given volume, the weight 
of charcoal required to yield a required reduction of pressure may be 
computed. The degree of vacuum obtained in a Dewar liquid-air container 
is discussed. 
The plumstone charcoal used by the author had a higher evacuating 
power than cocoanut charcoal. 
Reference is made to a colloidal silica of appreciable evacuating power, 
though, at —190° C. over four times as much of it is required (by volume) 
as of plumstone charcoal to attain the same result, and it acts more slowly. 
The very high degree of vacuum procurable by using a succession of 
charcoal bulbs is discussed, and it is shown that with a given weight of 
charcoal the reduction of pressure obtainable by division of the mass 
among a number of bulbs does not indefinitely increase with that number, 
but eventually reaches a maximum. 
(Issued separately March 14, 1922. 
VOL. XLII. 
3 
