438 Brown . — On the Preparation and 
It may be said generally that by means of three or four determinations 
performed as above the method will give the correct molecular value to 
within two units in the third decimal place. This degree of accuracy is 
thus somewhat greater than that claimed by Barger 1 for his method — viz. 
five units in the third decimal place. Each method has its own particular 
merits, but the present method has the great advantage for demonstration 
purposes that it measures osmotic pressure directly, i. e. by diffusion across 
a semipermeable membrane, and not indirectly through the effect of osmotic 
pressure of the solute on the vapour tension of the solvent. It thus illus- 
trates more clearly the functioning of the living cell as an osmotic unit. 
The carrying out of the above determinations requires about two days. 
In the course of the finer determinations which run over a period of about 
twenty-four hours the solutions must be protected against evaporation. It 
is advisable to have the unknown solution inside the osmometer during the 
longer periods, as it is thereby protected from evaporation, and in any case 
its concentration can be brought back to its original value. It is probable 
that an accuracy of 0-002 molar is about the best that can be obtained short 
of elaborate arrangements for preventing change in concentration through 
evaporation. It also approaches very close to the limits of accuracy which 
usually obtain in solutions prepared for quantitative work. 
In taking the above readings, the level of the meniscus was kept at 
about 10 cm. above the outside level. This head of water represents 
a molecular concentration of about 0-0005, and thus if greater accuracy is 
attempted than that claimed above, this factor will require to be allowed for. 
In order to bring back the meniscus in the capillary to the same point, 
and thus ensure that no alteration in concentration has taken place either 
by evaporation or by diffusion of water across the membrane, the osmometer 
is placed in water if the level has fallen, or, in the case where the level has 
risen, first washed on the outside with water, the excess of water drained off 
by means of filter-paper and the contents of the osmometer allowed to con- 
centrate to the required degree by evaporation of water through the mem- 
brane. This latter process can be repeated time and again without affecting 
the permeability of the membrane ; that is, no drying out of the membrane 
takes place at ordinary temperatures so long as one side of it is main- 
tained wet. 
Strictly speaking, these osmometers can only be used to determine the 
osmotic pressure of solutions the solutes of which do not diffuse across the 
membrane. As, however, the diffusibility of water is much greater than 
that of even the simplest electrolytes, considerable accuracy can be obtained 
even in the case of such substances. 
It will be noticed that the method of determining osmotic pressure 
here described is one of comparison and not of absolute measurement. For 
1 Journ. Chem. Soc. (Trans.), 1904, lxxxv, 286-324. 
