126 Proceedings of the Royal Society of Edinburgh. [Sess. 
though in this case the agreement is not so good. For hydrogen in 
cocoanut charcoal the following equations satisfy the data very exactly : — 
log. (|) = 1-92 --076®! (8) 
v = v Y + *25/> ...... (9) 
The argument does not take into account the probable thickening 
of the adsorbed layer as the pressure rises; if this effect were allowed 
for, k would become a variable depending upon the pressure. With 
gases below their critical temperature the effect in question will be 
considerable, but for gases such as hydrogen and nitrogen at 15° C. 
the proportional influence of such a correction must be small, and it 
has been disregarded. 
It will be seen (equations (4) to (7)) that the amount of the internal 
gaseous volume unoccupied by adsorbed films is, with nitrogen, about 
the same for silica and cocoanut charcoal. As the specific attraction 
between silica and nitrogen is, according to available evidence, less than 
that subsisting between cocoanut charcoal and nitrogen, the thickness 
of the adsorbed film will be less with the former than with the latter 
substance ; therefore it is probable that the average section of the 
internal passages is smaller in our colloidal silica than in the charcoal. 
This may in part be due to the absence in the silica of the relatively 
very large (microscopic) openings present in charcoal. The higher 
value of k in equation (9) points to the conclusion that, with cocoanut 
charcoal, the surface film of hydrogen is thinner than that of nitrogen. 
The straight-line relationships, exemplified by all but one of the 
graphs of fig. 2, should, we think, not be regarded as exceptions to the 
general law expressed by equations (2) and (3), but rather as special 
cases in which is of negligible influence; for if the term A-^ is 
inappreciable, the equations reduce to the straight line 
v = p(e A ° + k). . . . . ( 10 ) 
We propose to reserve for the present the consideration of adsorption 
under pressure of a gas, such as carbon dioxide, whose critical tempera- 
ture is above 15° C. — in this instance the temperature at which the 
experiments were carried out. It is sufficient to say that the double 
flexure of the curve, referred to above, is much more marked with 
such a gas, and, as might be expected, the slope of the curve increases 
rapidly as the pressure of liquefaction is approached. 
We wish to express our thanks to the Department of Scientific 
and Industrial Research for permission to publish this paper. 
