Brown . — On the Preparation and 
is recommended on account of the ease and rapidity with which the osmotic 
pressure of the unknown solution can be determined. 
The determinations now to be described were carried out with a 
moderately thin osmometer of 7 c.c. capacity, attached to a capillary tube 
of 1 mm. internal diameter. For testing purposes a half-molar solution of 
copper sulphate was prepared, and the accuracy of the method determined 
by comparing known dilutions of this stock solution with each other. 
In determining the osmotic strength of an unknown solution it is of 
great assistance to know certain constants appertaining to the particular 
osmometer. These should be determined when ^the osmometer is first 
used, for it will be found that a knowledge of them will greatly facilitate 
all future work with that particular osmometer. 
The rate of passage of water through the membrane is proportional to 
the difference of osmotic pressure of the liquids on opposite sides. If, 
therefore, the rate of passage corresponding to a known difference has been 
determined, it is possible to tell the whereabouts of an unknown concentra- 
tion by a single test of it against a known solution. The concentration sug- 
gested by this first reading can now be spt up and compared with the 
unknown solution. By this means a closer approximation is obtained at the 
second trial, and thus one rapidly arrives at the concentration of the un- 
known solution. 
With the particular osmometer under consideration, and with half- 
molar copper sulphate inside and water outside, the rise of the meniscus in 
1 hour is 3*3 cm. at 17 0 C. 
The rate of intake of water varies considerably with temperature, as is 
shown by the following figures : 
Rise of meniscus in 1 hour. 
Temperature. 
3-o 
3*3 
4.4 
Thus a constant temperature bath, if available, is advantageous, but if 
laboratory temperature does not vary more than a few degrees, and if 
a rough correction is made for this temperature effect, the rapidity with 
which one obtains a close approximation to the unknown solution is not 
seriously affected. 
In addition to the effect of temperature on the rate of endosmosis, 
account is to be taken of its more direct effect on the osmometer. In the 
present case the meniscus was found to rise 1*5 mm. for each i° C. 
In dealing with a solution of an electrolyte such as copper sulphate it 
is well to remember that the osmotic pressure is not directly proportional to 
the concentration of salt present. As it is a function of the number of 
molecules plus ions present, and as the degree of ionization depends on con- 
centration (in accordance with the law of mass action), one can easily prove 
