1921-22.] Prehensility : a Factor of Graseous Adsorption. 29 
curves to be drawn (figs. 2 and o) in which the abscissae are pressures 
and the ordinates the weights of gas (centigrams) adsorbed per gram 
of the substance. 
The materials named on fig. 2 are mostly anti-gas charcoals used in the 
war. The plumstone and cocoanut charcoals were activated by steaming, 
and the birch by long-continued heating in the presence of a low proportion 
of oxygen. The German charcoal is a pine charcoal impregnated with zinc 
and iron; the common wood charcoal (unactivated) was probably also 
made from pine. The activated anthracite is an American product. “ S.S. 
mixture ” is a briquetted mixture of coal and charcoal dusts prepared by 
Messrs Sutcliffe, Speakman & Co., Leigh. Silica (A) of fig. 3 was made 
from the hydrogel in the manner described elsewhere ; ^ it represents the 
Adsorption of Nitrogen at-!90°C. 
highest activation so far achieved with silica. Silica (B) is a colloidal 
silica prepared on a commercial scale at Baltimore, U.S.A., and dried, as 
the makers recommend, at about 120° C. The third curve of fig. 3 was 
obtained with a half-and-half mixture of powdered cocoanut charcoal 
and silica, the charcoal being mixed in when the silica was in the sol 
condition. 
The slope at the origin of any of the curves of figs. 2 and 3 is the 
prehensility of the substance concerned under the conditions of the test. 
The figures of the following table (p. 30) were ascertained from the 
curves ; two values of prehensility on dry hydrogen are also included. 
A problem met with in evacuation plant and in Dewar flasks is that 
of determining the degree of tenuity of the vacuum produced by means of 
a mass of charcoal cooled (usually) to liquid air temperature, the pressure 
having been reduced to a known value prior to the charcoal being put 
* Proc. Roy. Soc., A, 100, 1921, p. 88. 
