170 Proceedings of the Royal Society of Edinburgh. [Sess. 
tion of (1) the adsorbent and (2) the gas already adsorbed. The first 
effect will probably be represented by a function which diminishes to 
zero as more and more is adsorbed, if only because the sphere of molecular 
action of the adsorbent is attained. The second effect, on the other hand, 
may be represented by some function which will increase with the amount 
adsorbed, since the attracting “ layers ” will thicken. In the case of a 
vapour these “ layers” will be able to exist “outside” the adsorbing 
particles, so as to give the same final heat effect as condensation of vapour 
into liquid. (This effect, represented by the last portion of the curves 
above, will be absent in the case of a gas above its critical temperature, 
since the “ outside layers ” will never attain sufficient density to attract 
much of themselves after the sphere of molecular action of the adsorbent 
is passed.) 
The sum of a steadily diminishing function and a steadily increasing 
function may quite well present a minimum, as found in the observations 
above. 
It seems to the author that there should be a third term in the ex- 
pression for the heat effect, namely, one representing the change in energy 
