272 DIFFUSION, OSMOSIS, AND FILTRATION. 



maintained that cessation of motion of bacteria placed in solutions 

 indicates that the solution is isosmotic with the cell sap, in cases 

 where poisonous action can be excluded. 



The above physiological methods of measuring osmotic pressure are 

 of considerable interest, but, as already stated, their application is 

 decidedly limited ; and though, as will appear below, often of indirect 

 value, as giving information bearing on the permeability of cells and 

 membranes, they are not to be classed with the more accurate methods 

 of experimental physics. 



In the case of colloid solutions, it is not necessary to use pre- 

 cipitation membranes for the direct measurement of osmotic pressure, 

 for such material as vegetable parchment is, as a rule, impermeable to 

 colloids, and it moreover presents ceftain advantages in particular 

 cases, namely, those in which the colloidal substance is contaminated 

 by salts (e.g. albuminous solutions), since the salts can pass out, and 

 the determination is freer from the error of inclusion of the partial 

 pressure of these, unavoidable by a direct measurement by a copper 

 ferrocyanide membrane, or an indirect determination by lowering of 

 freezing-point. 



It is known that solutions of colloids of considerable concentration 

 exert very low osmotic pressure, though their exact measurement is 

 difficult. Picton and Linder, 1 in a direct measurement (by a copper 

 ferrocyanide membrane) of the pressure of a 4 per cent, solution 

 of colloidal arsenious sulphide, obtained a pressure of only 17 mm. of 

 water. Sabanejew 2 states that the lowering of freezing-point by silicic 

 acid is so small as to be within the limits of the method. With 

 albuminous solutions the difficulty of contaminating salts is almost 

 insuperable, and since the molecular weight of albumin is not known, 

 calculation is excluded. 



Sabanejew 3 investigated the lowering of freezing-point of water by 

 solution of egg albumin, and quotes a lowering of '02 C. for a 15*6 per 

 cent, solution, and -042 C. for a 30*35 per cent, solution, but since the 

 specimens held *4 to '66 per cent, of ash, the numbers are of 110 value. 

 Tamman 4 gives the difference in lowering of freezing-point of horses' 

 serum, produced by coagulation of the proteids by heat and removing 

 them, as only "006 C., which is in the region of the error of the method. 5 

 Dreser 6 and Koeppe 7 also state that the removal of proteid from 

 albuminous solutions does not affect the osmotic pressure, while 

 Ludeking 8 maintains that the boiling point of 40 per cent, solution 

 of gelatin is 100 C. 9 It is therefore uncertain whether proteids in 



1 Journ. Chem. Soc., London. 1895, vol. Ixvii. p. 63. 



2 Ber. d. deutsch. chem. Gesettsch., Berlin, 1890, Bd. xxiii. S. 87. 

 3 Ibid., 1891, Bd. xxiv. S. 558. 



4 Ztschr. f. physikal. Chem., Leipzig, 1896, Bd. xx. S. 180. 



5 Starling, on the other hand, quotes two experiments to prove that the osmotic 

 pressure of the proteids of serum can be directly measured. It is stated to be from 30 to 

 40 mm. of Hg. Journ. PhysioL, Cambridge and London, 1895, vol. xix. p. 323. Cf. also 

 next article, p. 308. 



6 Arch.f. exper. Path. u. PharmakoL, Leipzig, 1892, Bd. xxix. S. 314. 



7 Arch. f. d. ges. PhysioL, Bonn, 1896, Bd. Ixii. S. 571 (footnote). 

 8 Ann. d. Phys. u. Chem., Leipzig, 1888, Bd. xxxv. S. 552. 



9 A lowering of vapour pressure (raising of boiling point) is produced by solution 

 of a substance in a solvent, and the lowering of vapour pressure, like that of the 

 freezing-point, is proportional to the concentration. Wiillner, Ann. d. Phys. u. Chem., 

 Leipzig, 1858, Bd. ciii. S. 529 ; 1858, Bd. cv. S. 85 ; 1860, Bd. ex. S. 564 ; Tamman, 

 Ann. d. Phys. u. Chem., Leipzig, 1888, Bd. xxxiv. S. 299. 



