1921 - 22 .] Preheiisility : .a Factor of Gaseous Adsorption. 31 
(6) 
hence P ^ 2 ‘ 78 o-A^^ ^ ^ 
P Liv 6*2 
P 
It is convenient to speak of - (the ratio of the initial and final pres- 
sures) as the pressure-reduction factor, /. In almost all practical cases the 
0 
term is so small in comparison with that containing a that we may 
write ; 
For air ( 7 ) becomes : 
P 2 - 78 (tA 0 ^ 
p \jio 
V ~ 
L 
( 7 ) 
(8) 
If the initial temperature be 17 ° C., the substance dry cocoanut charcoal, 
air or nitrogen the gas, and if one gram be used per litre of space, / 
becomes 30 . That is to say, a pressure of OT mm. would be changed to 
one of 0’0033 mm. as the result of the charcoal being cooled to — 190 ° C. 
A metal vacuum vessel or container of 50 lbs. capacity, and designed 
for the storage and transport of liquid air or liquid oxygen, has a vacuous 
space of 6*27 litres, and is generally provided with about 200 grams of 
cocoanut charcoal or the same volume of some other charcoal. Applying 
these figures and the value of or given for cocoanut charcoal in Table I to 
equation (8), we find the pressure-reduction factor in this case to be 894 
(see Table II). Hence, if the initial pressure in the vacuum space be 
0*1 mm,, the pressure is reduced to about 0*00011 mm. when the container 
holds liquid air. At such a pressure the transference of heat by conduc- 
tion across the vacuum is only about one-tenth of the transference by 
radiation. 
Given that they contain a sufficient weight of dry charcoal, then, it is 
unnecessary to exhaust these metal Dewar flasks to the high degree of 
vacuum required for a glass flask. 
The following table gives the pressure-reduction factor, /' for a liquid 
air container in which there is either 200 grams of cocoanut charcoal or 
the same volume of other adsorbent. As the volumetric comparison is 
useful for other purposes also, a column is included of the apparent densities 
(cocoanut charcoal = 100), i.e. the relative weights of a litre of the granules, 
well shaken down. 
Though it is not claimed that any of these charcoals possessed their 
highest possible activation, the table reveals the superiority, in vacuum 
producing power, of British anti-gas fruitstone charcoal, despite the fact 
that the cocoanut charcoal had considerably the higher capacity at 
