AUGUST 20, 1914] 
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 
NATURE 
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especially physiological 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 in- 
terested 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 solu- 
tion according to a simple power law, and diminishes 
with rise of temperature; but there are many excep- 
tions 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 in- 
creases 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-§ 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 diffi- 
cult 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 places and becomes much 
thicker at others, intermediate thicknesses to these 
being apparently unstable and unable to exist. As 
helping towards an explanation 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 in- 
stability the process is irreversible, so that the appli- 
cation 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 
NO. 2338, VOL. 93] 
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643 
| mutually attract one another, while at very close dis- 
tances 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 
distance 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 re- 
pulsive; then, as the distance apart is ‘supposed to 
increase, the force of repulsion diminishes, and after 
passing through zero value becomes positive or attrac- 
tive; 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 be- 
tween the particles in the adsorption layer, through 
the particles becoming so crowded in places as to 
reduce their mutual distances to the stage when re- 
pulsion 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 ad- 
sorbed from a given solution. This obviously necessi- 
tates 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 tempera- 
ture and strength of solution. The normal condensa- 
tion 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 precipi- 
tated 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 
as being saved from thirst in a place where the water 
was charged with alkali by filtering the water through 
bags of sand. Whether this is a practical thing or 
not is doubtful, or even if it has ever been tried; for 
it is only the first part of the liquid to come through 
the filter which is purified, and very soon the surface 
has taken up all the salt it can adsorb, and after that, 
of course, the solution comes through intact. It is 
