OSMOSIS. 269 



Blagden l discovered the fact that the freezing-point of a solution is 

 lower than that of the solvent, and that the lowering of freezing-point 

 is proportional to the concentration of the solution. Kudorff, 2 Coppet, 3 

 and Kaoult 4 have since more thoroughly investigated the matter. If, 

 therefore, we know the lowering of the freezing-point of water, produced by 

 the addition of a gramme-molecule to the litre (1'89 C.), and the osmotic 

 (or gas) pressure at C. corresponding to this (22*35 atmospheres), it is 

 merely a matter of simple proportion to calculate the pressure at 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 5 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. t; 



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. 8 

 If the ferrocyanide solution have the same osmotic pressure as the 

 copper solution, no schlieren will be produced, and there will be no 

 change in 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. Ixxviii. p. 277. 



2 Ann. d. Phys. u. Chem., Leipzig, 1861, Bd. cxiv. S. 63 ; 1862, Bd. cxvi. S. 55 ; 1871, 

 Bd. cxlv. S. 599. 



s Ann. de chim., Paris, 1871, Ser. 4, tome xxiii. p. 366 ; 1872, tome, xxv. p. 502 ; 1872, 

 tome xxvi. p. 98. 



4 Ibid., Paris, 1884, Se'r. 6, tome ii. p. 66; Compt. rend. Acad. d. sc., Paris, 1882, 

 tome xcv. p. 1030. 



5 Ztschr. f. physical. Chem., Leipzig, 1888, Bd. ii. S. 638. 



" Loomis, Ann. d. Phys. u. Chem., Leipzig, 1894, Bd. Ii. S. 500; Jones, Ztschr. f. 

 nkal. Chem., Leipzig, 1893, Bd. xi. S. 110 ; Raoult, ibid., 1892, Bd. ix. S. 343. 

 7 Ztschr. f. physikal. Chem., Leipzig, 1888, Bd. ii. S. 415. 

 8 Ann. d. Phys. u. Chem., Leipzig," 1867, Bd. cxxxi. S. 33. 



