286 TRANSACTIONS OF SECTION A. 
deny the possibility of tapping this energy despite the ingenious theories of 
relativity which have been put forward to explain matters away. There is no 
doubt but that up to the present nothing hopeful has been accomplished towards 
reaching this energy and there are grave difficulties in the way; but ‘Relativity ’ 
is, as it were, merely trying to remove the lion in the path by laying down the 
general proposition that the existence of lions is an impossibility. The readiness 
with which the fundamental hypothesés of ‘ Relativity ’ were accepted by many 
is characteristic of present-day Physics, or perhaps, more correctly speaking, 
is an exaggerated example of it. 
Such an acceptance as this could hardly be thought of as taking place half-a- 
century ago, when a purely dynamical basis was expected for the full explanation 
of all phenomena, and when facts were only held to be completely understood if 
amenable to such treatment; while, if not so, they were put temporarily into 
a kind of suspense account, waiting the time when the phenomenon would 
succumb to treatment based on dynamics. 
Many things, perhaps not the least among them radio-activity, have conspired 
to change all this and to produce an attitude of mind prepared to be content with 
a much less rigid basis than would have been required by the Natural Philo- 
sophers of a past generation. These were the sturdy Protestants of Science, to 
use an analogy, while we of the present day are much more catholic in our 
scientific beliefs, and in fact it would seem that nowadays to be used to anything 
is synonymous with understanding it. 
Leaving, however, these interesting questions, I will confine my remarks to a 
rather neglected corner of physics, namely, to the phenomena of Absorption and 
Adsorption of solutions. ‘The term Adsorption was introduced to distinguish 
between Absorption which takes place throughout the mass of the absorbing 
material and those cases in which it takes place only over its surface. If, for 
instance, glass, powdered so as to provide a large surface, is introduced into a 
solution of a salt in water, we have in general some of the salt leaving the body 
of the solution and adhering in one form or other to the surface of the glass. It 
is to this the term Adsorption has been applied. Physicists have now begun to 
take up the question seriously, but it was to Biologists, and especially Physio- 
logical Chemists, that most of our knowledge of the subject in the past was due, 
the phenomenon being particularly attractive to them, seeing that so many of the 
processes they are interested in take place across surfaces. 
As far as investigations already made go the laws of Adsorption appear to be 
very complicated, and no doubt many of the conflicting experimental results 
which have been obtained are in part due to this, workers under somewhat 
different conditions obtaining apparently contradictory effects. 
On the whole, however, it may be said that the amount adsorbed increases 
with the strength of solution according to a simple power law, and diminishes 
with rise of temperature; but there are many exceptions to these simple rules. 
For instance, in the case of certain sulphates and nitrates the amount adsorbed 
by the surface of, say, precipitated silica only increases up to a certain critical 
point as the strength of the solution is increased. Then further increase in the 
strength of the solution causes the surface to give up some of the salt it has 
already adsorbed, or the amount adsorbed is actually less now than that adsorbed 
from weaker solutions. Beyond this stage for still greater concentrations of the 
solutions the amount adsorbed goes on increasing as before the critical point was 
reached. 
There is some reason for thinking that there are two modes in which the salt 
is taken up or adsorbed by the solid surface. The first of them results from a 
simple strengthening of the solution in the surface layers; the second, which 
takes place with rather stronger concentrations, is a deposition in what is 
apparently analogous to the solid form. It would seem that the first reaches out 
from the solid surface to about 10-8 cm.—which is the order of the range of 
attraction of the particles of the solid substance. 
The cause of the diminution in the adsorption layer at a certain critical value 
of the concentration is difficult to understand. Something analogous has been 
observed by Lord Rayleigh in the thickness of layers of oil floating on the surface 
of water. As oil is supplied the thickness goes on increasing up to a certain 
point ; beyond this, on further addition of oil, the layer thins itself at some 
