TRANSACTIONS OF SECTION B. 361 
further absorption of 79 calories takes place, so that a gram of liquid water at the 
freezing-point contains the heat energy of 215°5 calories. ‘The fact that water has 
the high vapour pressure of 4:6 mm. of mercury at the freezing-point is probably 
a result of this enormous store of energy. As a liquid, therefore, it is natural to 
expect that its molecules will exhibit effects proportionate. to this great store of 
energy. This expectation appears to be realised when we consider not only its 
properties as the universal solvent, but its osmotic and diffusive energy in solutions 
in which it is the solvent. 
To complete the comparison it is only necessary to calculate the heat energy of 
gold at 0°. Taking its specific heat as 0-032, a gram of gold at 0° contains 8:7 
calories. A gram molecule, therefore, contains in round numbers 1,700 calories as 
compared with 3,880 calories in a gram molecule of water. 
Taking into consideration not only this greater store of energy, but also the 
much smaller cohesive force of water as compared with the majority of solid 
solutes, there can be no doubt that the active rédle in aqueous solutions of this 
type must be assigned to the solvent, not to the solute molecules. 
This leads to the important conclusion that the energy of solution, of diffusion, 
and of osmosis is due, not to the imaginary gaseous energy of the solute, but to the 
actual liquid energy of the solvent molecules. When this conclusion is reached a 
new physical explanation of these phenomena is in our hands, and we are relieved 
from the strain to the imagination involved in the application of the gas theory to 
solutions of non-volatile solids. 
This transference of the active réle to the solvent molecules does not in any 
way affect the well-established conclusions based on the laws of thermo-dynamics 
as to the energy relations in these phenomena, for it has always been recognised 
that these conclusions have reference to the average conditions prevailing in large 
collections of relatively minute units. Wherever the gas analogy has appeared to 
hold it has not necessarily involved more than this, that the observed effects are 
in proportion to the number of these minute units in a given volume. 
In applying the gas theory to the physical explanation of osmotic pressure it 
has been the custom to regard this pressure as directly due to the bombardment of 
the semi-permeable membrane by the solute molecules. But this conception com- 
pletely ignores the fact that the pressure developed is a hydrostatic, not a gaseous 
pressure, and that the hydrostatic pressure results directly from the penetration of 
the solvent molecules from the other side of the partition, 
It appears to me more natural to abandon the gas analogy altogether, to 
regard the molecules as in the solid and liquid condition proper to their tempera-~ 
ture, and to apportion to them their respective parts in the active changes accord- 
ing to their obvious endowment of energy. 
Applying this view to the case of a solution and a solvent separated by a 
semi-permeable membrane, it is seen that the pressure rises on the solution side, 
because the pure solvent molecules on the other side have some advantage for the 
display of their energy over the similar molecules in the solution. This effect vm 
its most general form may be attributed to the dilution of the solvent by the solute 
molecules. In cases where the osmotic pressure appears to obey Boyle’s law the 
effect is exactly measured by the number of solute molecules per unit volume. 
But the facts of this position are in no way changed if the effect is taken to be due 
to the activity of an equal number of solvent molecules, for we then see that eacls 
solute molecule by cancelling the activity of one solvent molecule on the solution, 
side permits a solvent molecule from the other side to enter the solution. 
What the exact mechanism of this cancellation is there is at present no 
evidence to show, and the caution originally given by Lord Kelvin with reference. 
to the undue forcing of the gas analogy must also be applied to the suggestion now: 
put forward. But as a means of making the suggestion a little more clear I give. 
here a simple diagram on which A represents a single perforation in a semi-. 
permeable membrane, Pr, on both sides of which there is only pure solvent. For: 
the sake of clearness the molecules are shown only as a single row. Normally 
there wil] be no passage of solvent molecules ,from side to side, for the average 
kinetic energy of the molecules on both sides is equal. This state of equilibrium 
