OSMOSIS. 269 
Blagden 1 discovered the fad that the freezing-point of a solution is 
lower than thai of the solvent, and that the lowering of freezing-point 
is proportional to the concentration of the solution. Riidorff, 2 Coppet, 3 
and Kauult l have since more thoroughly investigated the matter. If, 
therefore, we know the Lowering of the freezing-point of water, produced by 
l ho addition of a gramme-molecule to the litre (1 "89 0. ), and the osmotic; 
(in' gas) pressure at < '. corresponding to this (22"35 atmospheres), it is 
merely a matter of simple proportion to calculate the pressure at 0° C. 
corresponding to any given lowering of freezing-point, and from that to 
obtain the pressure at any other temperature by the law of Charles. 
Many pieces of apparatus have been devised for measuring the 
lowering of the freezing-point, but that of Beckmann s is in most 
general use. Unfortunately, the method does not yield concordant 
results in the hands of different observers (when aqueous solutions 
are used) within about 005° C, which corresponds to an osmotic pressure 
of about 50 mm. of mercury at the temperature of the body (37° C), and 
is hence of little value for the correct estimation of small differences of 
osmotic pressure in the aqueous solutions to which the physiologist 
confines his attention.'' 
An optical method has been used by Tamilian. 7 If a drop of 
solution of potassium ferrocyanide is allowed to fall into a solution 
of copper sulphate, a so-called " Traube cell " is formed, the ferrocyanide 
solution within which is separated from the copper sulphate solution 
outside by a precipitation membrane of copper ferrocyanide, through 
which osmotic interchange can take place. 
If the internal solution be of higher osmotic pressure than the 
external, water passes from the copper solution outside into the 
cell, and the copper solution immediately round about the cell, being 
raised in concentration, tends to sink. In the reverse case, by dilution 
of the layer round the cell, an upward current is started. There are 
thus produced differences in the refractive index of the layer of solution 
against the outside of the cell, in contrast to the rest of the copper 
solution. These are easily detected by the Topler Schlierenapparat* 
If the ferrocyanide solution have the same osmotic pressure as the 
copper solution, no schlieren will be produced, and there will be no 
change iu refraction. Now, since the total osmotic pressure is the sum 
of the partial pressures, a third substance, not reacting with the 
membranogens, may be added to the solution of one of them, and the 
concentration of the other, isosmotic with the mixture, determined by the 
method. Since the osmotic pressure of the solution of the mem- 
branogens, to which the third substance was added, is directly 
measurable, it is obvious that the partial pressure of the added 
substance can be measured. 
1 Phil. Trans., London, 1788, vol. lxxviii. p. 277. 
2 Ann. d. Phys. u. Chum., Leipzig, 1861, Bd. cxiv. S. 63 ; 1862, Bd. cxvi. S. 55 : 1871, 
Bd. cxlv. S. 599. 
8 Arm. de chim., Paris. 1871, Ser. 4, tome xxiii. p. 366 ; 1872, tome, xxv. p. 502 ; 1872, 
tome xxvi. p. 98. 
4 Ibid., Paris, 1884, S^r. 6, tome ii. p. 66; Compt. rend. Acad. d. sc., Paris, 1882, 
tome xcv. p. 1030. 
5 Ztschr.f. physikal. Chem., Leipzig, 1888, Bd. ii. S. 638. 
6 Loomis, Ann. d. Phys. it. Chem., Leipzig, 1894, Bd. Ii. S. 500; Jones, Ztschr. /'. 
physikal. Chem., Leipzig, 1893, Bd. xi. S. 110 ; Raoult, ibid., 1892, Bd. ix. S. 343. 
1 Ztschr. f. physikal. Chem., Leipzig, 1888, Bd. ii. S. 415. 
8 Ann. d. Phys. u. Chem., Leipzig, 1867, Bd. cxxxi. S. 33. 
