Permeability 3 
of pure solvent and solution respectively, s the specific volume of 
the vapour and v s is the increase in volume of a large mass of solution 
on addition of unit mass of solvent. For other formulae, more or 
less exact, Findlay’s book may be consulted, or the original papers 
of Arrhenius (1889), Berkeley and Hartley (19066), Porter (1907, 
1908), Callendar (1908), and Berkeley, Hartley and Burton (1908). 
The actual method of measurement of the vapour pressure depends 
on bubbling an indifferent gas through the solution, the amount of 
vapour taken up by the gas being proportional to the vapour pressure. 
In the original experiments of Walker (1888) the gas is allowed to 
bubble first through the solution and then through the pure solvent. 
The amount of vapour taken up in the second operation is propor¬ 
tional to the difference between the vapour pressures of pure solvent 
and solution. 
3. Freezing Point Method. As is well known the presence of a 
dissolved substance lowers the freezing point of a solution. A number 
of formulae have been derived also in this case to express the relation 
between lowering of freezing point and osmotic pressure, for a dis¬ 
cussion of which the reader is again referred to Findlay. 
4. Boiling Point Method. Just as determinations of the lowering 
of the freezing point of a solution due to a dissolved substance can 
be used to calculate the osmotic pressure of a solution, so the eleva¬ 
tion of the boiling point resulting from the presence of a solute can 
be used to the same end. The method has, however, been little em¬ 
ployed for this purpose. 
5. Critical solution temperature method. Certain liquids are 
miscible in all proportions only above a certain temperature. Below 
this temperature they are miscible only to a limited extent, and con¬ 
sequently if a liquid is cooled below this temperature one of the 
liquids separates out as a distinct phase. This critical solution tem¬ 
perature is altered by dissolved substances in proportion to the 
osmotic pressure of the solute. A method of measuring osmotic 
pressure based on the work of Timmermans (1907) in this relation 
has been employed for-physiological purposes by Atkins and Wallace 
(1913) to whose work further reference may be made. 
Methods for comparing osmotic pressures by the use of living 
cells and tissues were devised by de Vries (1884 a, h). These are 
the plasmolytic methods and the tissue tension method. They 
will be more conveniently described in a later chapter dealing with 
the water relations of the plant cell. 
