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.