PRESIDENTIAL ADDRESS. 287 
places and becomes much thicker at others, intermediate thicknesses to these 
being apparently unstable and unable to exist. As helping towards an explana- 
tion of the diminution in the adsorption layer, we may suppose that as the 
strength of the solution is increased from zero, the adsorption is at first merely 
an increased density of the solution in the surface layer. For some reason, 
after this has reached a certain limit, further addition of salt to the solution 
renders this mode of composition of the surface layers unstable, and there is a 
breaking up of the arrangement of the layer with a diminution in its amount. 
We may now suppose the second mode of deposition to begin to show its effect 
with a recovery in the amount of the surface layers and a further building up of 
the adsorption deposits. 
On account of passing through this point of instability the process is 
irreversible, so that the application of thermo-dynamics to the phenomenon of 
adsorption is necessarily greatly restricted in its usefulness. 
A possible cause of the instability in the adsorption layer which occurs at 
the critical point may be looked for in the alternations in the sign of the mutual 
forces between attracting particles of the kind suggested by Lord Kelvin and 
others. Within a certain distance apart—the molecular range—the particles of 
matter mutually attract one another, while at very close distances they obviously 
must repel, for two particles refuse to occupy the same space. At some inter- 
mediate distances the force must pass through zero value. It has for various 
reasons been thought that, in addition, the force has zero value at a second dis- 
tance lying between the first zero and the molecular range, with accompanying 
alternations in the sign of the force. Thus, starting from zero distance apart 
of the particles, the sign of the force is negative or repulsive; then, as the dis- 
tance apart is supposed to increase, the force of repulsion diminishes, and after 
passing through zero value becomes positive or attractive; next, as the distance 
is increased the force diminishes again, and after passing through a second zero 
becomes negative for a second time; finally, the force on passing through a third 
zero becomes positive, and is then in the stage dealt with in capillary and other 
questions, 
As an instance of where these alternations of sign seem to be manifest, may 
be mentioned the case of certain crystals when split along cleavage planes. The 
split often runs along further than the position of the splitting instrument or 
inserted wedge seems to warrant. This would occur if the particles on either 
side of the cleavage plane were situated at the distance apart where the force 
between them was in the first attractive condition, for then, on increasing the 
distance between the particles by means of the wedge, the force changes sign and 
becomes repulsive, thus helping the splitting to be propagated further out. 
Assuming that a repulsive force can supervene between the particles in the 
adsorption layer, through the particles becoming so crowded in places as to 
reduce their mutual distances to the stage when repulsion sets in, we might 
expect that an instability would be set up. 
As already stated, a rise in temperature reduces in general the amount 
adsorbed, but below the critical point the nitrates and sulphates are exceptional, 
for rise in temperature here increases the amount adsorbed from a given solution. 
This obviously necessitates that the isothermals cross one another at the critical 
point in an Adsorption-Concentration diagram. This may perhaps account for 
some observers finding that adsorption did not change with temperature. We 
have another exception to the simple laws of adsorption in the case of the alkali 
chlorides; this exception occurs under certain conditions of temperature and 
strength of solution. The normal condensation into the surface layer is reversed 
and the salt is repelled into the general solution instead of being attracted by 
the surface. In other words, it is the turn of the other constituent of the 
solution, namely, the water, to be adsorbed. 
It is a very well known experiment in adsorption to run a solution such as 
that of permanganate of potash through a filter of sand, or, better, one of 
precipitated silica, so as to provide a very large surface. The first of the solution 
to come through the filter has practically lost all its salt owing to having been 
adsorbed by the surface of the sand. 
I was interested in finding a few months ago that Defoe, the author of 
‘ Robinson Crusoe,’ in one of his other books, depicts a party of African travellers 
