48 Proceedings of the Royal Society of Edinburgh. [Sess. 
VI. — The Adsorption Isotherm at Low Concentrations. By 
A. M. Williams, M.A., D.Sc. Communicated by Professor James 
Walker, F.R.S. 
(MS. received October 29, 1918. Read December 2, 1918.) 
Several formulae have been proposed to express the adsorption equililibrium 
between the amount adsorbed a and the equilibrium concentration c. That 
most extensively employed was apparently first used by Kiister,* and is 
based on the increasing positive adsorption of the first portion of the (c, a) 
curve in the case of solutions (see fig. 1). The graph of (log c, log a) is 
found to be approximately a straight line with a gradient less than unity, 
and hence 
log a = log a 0 + i log c 
or 
a - a Q Cn. 
In the case of gases the pressure p usually replaces c in the formula, which 
is then written 
_i 
a = a 0 p n > 
With gases, like hydrogen at ordinary temperatures and other gases 
at high temperatures the value of the exponent appears to be unity for 
moderate pressures. The mean value of ^ over the ordinary range of 
pressures — that is, up to atmospheric — rises with the temperature, as is 
shown, for example, by Travers *(■ for the adsorption of carbon dioxide by 
charcoal. 
Table I, 
t 
2 
n 
- 78° C. 
0-13 
0 
•30 
35° 
•46 
61° 
•48 
100 
•52 
* See Ostwald, Lehrbuch der allgemeinen Ghemie (1906), II, iii, p. 253. 
f Proc. Roy. Soc. (1906), A, 78, p. 9. 
