290 TRANSACTIONS OF SECTION A. 
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. Certainly 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 attri- 
butable 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 pressure 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 produced 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 
quantity 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 
mechanism 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 bombard- 
ment 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 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 hydro- 
static 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 forwards 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 superambient vapour 
and back from it into the solution. 
The processes are very similar, namely, equal molecular transfer to and fro 
across the respective surfaces of separation. 
Thus we may in the case of osmotic equilibrium attribute the phenomenon 
with Callender to evaporation, but not evaporation in its restricted sense, from 
a free surface of liquid, but as we have seen from a saturated collodial surface 
into the solution. This process might perhaps be better referred to as molecular 
emigration, the term migration being already a familiar one in connection with 
liquid phenomena. 
The following Report and Papers were then read :— 
1. Report of the Committee to Aid in Establishing a Solar Physics 
Observatory in Australia.—See Reports, p. 74. 
