AUGUST 20, 1914] 
NATURE 
645 
posit, thus necessitating an increase in pressure for 
equilibrium. If either or both of these effects really 
exist, it would seem to require that the pressure 
should be higher for equilibrium of the molecular 
surface transfer than if there were no adsorption 
layer and the unaltered solution were to touch the 
medium, but at the same time it should be remem- 
bered that there is a second surface where equilibrium 
must also exist—that is, the surface of separation of 
the adsorption layer and the solution itself. It is 
just possible that the two together cancel each other’s 
action. 
Quantitative determinations of absorption by solid 
media from solution are hard to carry out, but with 
a liquid medium it is not so difficult. Ether con- 
stitutes an excellent semi-permeable medium for use 
with sugar solution, because it takes up or dissolves 
only a small quantity of water and no sugar. A 
series of experiments using these for medium and 
solution has shown (1) that the absorption of water 
from a solution diminishes with the strength of the 
solution; and (2) that the absorption of water for any 
given strength of solution increases with the pressure. 
This increase with pressure is somewhat more rapid 
than if it were in proportion to the pressure. On the 
other hand, from pure water ether absorbs in excess 
. of normal almost in proportion to the pressure. Cer- 
tainly this is so up to 100 atmospheres. This would 
go to confirm the suggestion already made that the 
departure from proportionality in the osmotic pressure 
is attributable to absorption. 
By applying pressure ether can be thus made to 
take up the same quantity of water from any given 
solution as it takes up from pure water at atmospheric 
pressure. It is found by experiment that this pres- 
sure is the osmotic pressure proper to the solution in 
question. 
Decidedly the most interesting fact connected with 
the whole question of osmotic pressure, the behaviour 
of vapour pressures from solution, and the equilibrium 
of molecular transfer of solutions with colloids, is 
that discovered by van ’t Hoff, that the hydrostatic 
pressure in question is equal to what would be pro- 
duced by a gas having the same number of particles 
as those of the introduced salt. Take the case of a 
mass of colloid or semi-permeable medium placed in 
a vessel of water; the colloid when in equilibrium at 
atmospheric pressure holds what we will call the 
normal moisture. By increasing the pressure this 
moisture can be increased to any desired amount. 
Now, on introducing salt the moisture in the colloid 
can be reduced at will. The question is, what quan- 
tity of salt must be introduced just to bring back the 
amount of the moisture in the colloid to normal? 
Here we get a great insight into the internal mechan- 
ism of the liquid state. The quantity of salt required 
turns out to be, approximately at least, that amount 
which if in the gaseous state would produce the 
pressure. So that normality can be either directly 
restored by removing the pressure or indirectly by 
introducing salt in quantity which just takes up the 
applied pressure. That this is so naturally suggested 
that the salt, although compelled to remain within 
the confines of the liquid, nevertheless produces the 
same molecular bombardment as it would were it 
in the gaseous state, though, of course, the free path 
must be viewed as enormously restricted compared with 
that in the gaseous state. 
Many have felt a difficulty in accepting this view 
of a molecular bombardment occurring in the liquid 
state, but of recent years much light has been 
thrown on the subject of molecular movements in 
liquids, especially by Perrin’s work, so that much of 
NO. 2338, VOL. 93] 
the basis of this difficulty may be fairly considered as 
now removed. 
Quite analogous to the reduction from the normal 
of the moisture held by a semi-permeable medium 
brought about by the addition of salt to the water, is 
the reduction in the vapour pressure arising from the 
presence of a salt'in the water. The vapour pressure 
is likewise increased by the application of hydrostatic 
pressure, which may be effected by means of an inert 
gas. In both cases the hydrostatic pressure which 
must be applied to bring back to normality is equal 
to that which the added salt would exert if it were 
in the state of vapour or, in other words, the osmotic 
pressure. 
The two cases are really very similar. In both 
there is equal molecular transfer backwards and for- 
wards across the bounding surface. In the one a 
transfer from that solution to the semi-permeable 
medium and back from it into the solution. In the 
other a transfer from the solution into the super- 
ambient vapour and back from it into the solution. 
The processes are very similar, namely, equal mole- 
cular transfer to and fro across the respective surfaces 
of separation. 
Thus we may in the case of osmotic equilibrium 
attribute the phenomenon with Callendar to evapora- 
tion, but not evaporation in its restricted sense, from 
a free surface of liquid, but as we have seen from a 
saturated colloidal surface into the solution. This 
process might perhaps be better referred to as mole- 
cular emigration, the term migration being already a 
familiar one in connection with liquid phenomena. 
SECTION B. 
CHEMISTRY. 
OprNING ADDRESS BY PRoF. WILLIAM J. Pope, M.A., 
Pi Da kh RIS, | PRESIDENTNEON LHe SEG TON: 
Tue British Association has been firmly established 
as one of the institutions of our Empire for more than 
half a century past. The powerful hold which it has 
acquired probably arises from the welcome which 
every worker in science extends to an _ occasional 
cessation of his ordinary routine—a respite during 
which the details of the specific inquiry in hand may 
be temporarily cast aside, and replaced by leisurely 
discussion with colleagues on the broader issues of 
scientific progress. 
The investigator, continually occupied with his own 
problems and faced with an ever-increasing mass of 
technical literature, ordinarily finds little time for 
reflection upon the real meaning of his work; he 
secures, in general, far too few opportunities of con- 
sidering in a philosophical sort of way the past, 
present, and future of his own particular branch of 
scientific activity. It is not difficult to form a fairly 
accurate survey of the position to which chemistry 
had attained a generation ago, perhaps even a few 
years ago; probably no intellect at present existing 
could pronounce judgment upon. the present position 
of our science in terms which would commend them- 
selves to the historian of the twenty-first century. 
Doubtless even one equipped with a complete know- 
ledge of all that has been achieved, standing on the 
very frontier of scientific advance and peering into the 
surrounding darkness, would be quite incompetent to 
make any adequate forecast of the conquests which 
will be made by chemical and physical science during 
the next fifty years. At the same time, chemical 
history tells us that progress is the result in large 
measure of imperfect attempts to appreciate the pre- 
sent and to forecast the future. I therefore propose to 
