Methods for Determining Osmotic Pressures. 167 
in which the change in the freezing point is measured, not by a 
thermometer, but by means of a galvanometer. The form of 
apparatus used requires 2'5-c.c. to 5-c.c. of liquid. The authors used 
their method to estimate the osmotic pressure of the expressed sap 
of the leaves of a number of trees. Dixon 1 has also estimated the 
osmotic pressure in leaves by placing shoots in a closed vessel con¬ 
taining some gas under high pressure and observing the pressure 
at which the leaves collapsed, i.e., he found an external gas-pressure 
which balanced the internal pressure in the cells. 
II.—Application of Barger’s Method of Determining Mole¬ 
cular Weights to the Estimation of the Osmotic Pressure 
of the Cell Sap of Plants. 
In view of the fact that all the purely physical methods of 
determining the osmotic pressure of the cell sap of plants hitherto 
employed require a comparatively large amount of cell sap or some¬ 
what elaborate apparatus, I thought it would be of interest to put 
on record a method I have used to determine the osmotic pressure 
of the cell sap of certain salt marsh and other succulent plants. 
The method is a vapour pressure one and was invented by 
Barger 2 for estimating the molecular weights of small quantities of 
organic substances. The vapour pressure of the solution of unknown 
strength is compared with those of solutions of known strength. 
The vapour pressure of any solvent is lowered by the addition of 
any substance, and the amount by which it is lowered is proportional 
to the weight of substance present. As the osmotic pressure of 
any solution depends also on the amount of substance in solution 
and increases according to the amount of substance dissolved, the 
osmotic pressure of any solution varies inversely as the vapour 
pressure. 
Barger states in his paper that the idea originated from an 
observation made during some experiments, on the adaptation of 
fungi to salt solutions, carried out in the late Professor Errera’s 
laboratory in Brussels. The fungi were grown in hanging drops of 
solutions, the sides of the damp chambers being of cardboard kept 
moist with distilled water. The drops of solution were found to 
increase in size. This increase in size was explained by Professor 
1 “On the Osmotic Pressure in the Cells of Leaves.” Proc. Roy. Irish 
Acad., Ser. 3, Vol. 4, 1896, p. 61. 
2 “A Microscopical Method of Determining Molecular Weights.” Trans. 
Chem. Soc., Vol. 85, 1906, p. 287. 
