498 
PROFESSOR J. J. THOMSON ON SOME APPLICATIONS OF 
Hence we see that the density of the saturated solution depends upon the extent 
of surface of the liquid, so that, if we alter the surface, we may either deposit or absorb 
salt. Effects of this kind have, I believe, been observed by chemists. 
Let us now proceed to investigate the effects of pressure on the solubility. Let us 
suppose that the mixture of salt and solution is under a pressure y> per unit area. The 
effect of the pressure will be due to two causes. The first is the change of volume 
which accompanies solution, and which causes the process of solution to be accompanied 
by an expenditure or absorption of work ; the second cause is that the energy due to 
the strain in unit mass of the salt is not, in general, the same as that of an equal mass of 
the solution, so that there will be a change in the potential energy when the salt 
dissolves. If k be the bulk modulus for the salt, k for the solution, the rest of the 
notation being the same as before, then the change in the potential energy due to 
strain is, when unit mass of salt dissolves, 
{4- + j-, - - - j ; 
|_a :p clq k k aq p cr/cj 
the increase in the potential energy due to the change in volume is 
d 
so that we have, by equation (37), 
p y o +»'). 
^ i t / d 1 i dw \ d . /\ * -i o / f , d 1 i d 1 1 \ /a o\ 
00 [~ P +wf J d?~ P + v W) =P^ v + v ) + i^[^ v + < 42 > 
or, if the change due to pressure be the same as the change due to an increase Sd in 
the temperature, we have 
1 
P 
(43) 
Thus we see that the effect of pressure consists of two parts, one of which is propor¬ 
tional to the pressure, and the other to the square of it. It will be well to try to 
gain some idea of the relative magnitude of these effects. Let us suppose that the 
pressure is n atmospheres, that is, in CIG.S. units, n X I0 6 . We do not know 
accurately the value of the second term on the left-hand side of equation, but, since k 
for water is about 2 X 10 10 , the multiplier of yd will be of about the order 10 -11 , so 
that the ratio of the second term to the first will be of the order 
n x 10 -5 
JWW 
