OSMOSIS. 271 



freezing-point method, it is found to be higher (about 1'89). Other 

 indices of " isotony " than the plasmolysis of the vegetable cell have 

 been used by physiologists. 



Hamburger 1 has used red blood corpuscles. If these are placed 

 in solutions of substances which do not penetrate, and which do not 

 act chemically upon them, an index of the entrance of water into 

 the substance of the corpuscle is presented by the setting free of 

 haemoglobin, which is recognisable in the solution. In a solution of lower 

 osmotic pressure than the corpuscle-contents (hypoisotonic solution), 

 water enters the corpuscles and the solution is reddened. In a solution 

 of higher osmotic, pressure than the contents (hyperisotonic solution), 

 water is extracted from the corpuscles, they shrivel and sink, and the 

 solution retains its original colour. Two limiting solutions are thus 

 obtainable, and the mean concentration of the two is taken as that 

 isosmotic with the contents of the corpuscle. 



The method has very considerable limits in practice, for not only 

 is it obviously restricted to colourless solutions, but it can also only 

 give results approaching the truth, in cases where the substance in 

 solution does not penetrate ; and, as indicated by Gryns, 2 who has 

 criticised the method very severely, the red corpuscles are penetrable 

 by a very large number of substances. 3 



Another blood corpuscle method is that of the haematokrit. 4 Here 

 the gauge of entrance or exit of water from the corpuscles is the volume 

 they occupy, in a graduated capillary tube, after having been centrifu- 

 galised with the solution. The volume of the corpuscles is dependent 

 on the osmotic pressure of the solution in which they are placed (provided 

 the dissolved substance does not penetrate), and if equal volumes of the 

 same blood specimen, contemporaneously centrif ugalised in two solutions 

 of different substances, give the same volume of corpuscles, those solutions 

 have the same osmotic pressure. By centrifugalising a given volume 

 of a blood sample in a series of solutions of a substance not penetrating 

 corpuscles (cane sugar), of different and known osmotic pressure, in 

 separate tubes, at the same time as an equal volume of the same blood 

 treated with the solution of the substance to be investigated, a final 

 comparison of the length of the " threads " of corpuscles in the tubes 

 gives a gauge of the osmotic pressure of the solution. 



By centrifugalising blood in a pipette, previously oiled (cedar oil) 

 to prevent clotting, measuring the length of the " thread," and 

 comparing with the same, blood treated with sugar solutions of known 

 osmotic pressure, the pressure of the plasma is determinable and is 

 found to vary, rising after meals, and especially after the ingestion 

 of solutions of salt (Koeppe). 



A bacterial method even has been used by Wladimiroff, 5 who has 



1 Arch. f. Anat. u. Physiol., Leipzig, 1886, Phys. Abth., S. 476; 1887. S. 31 ; 

 Ztschr. f. physical. Chem., Leipzig, 1890, Bd. vi. S. 319. 



2 Arch.f. d. ges. Physiol., Bonn, 1896, Bd. Ixiii. S. 86. 



3 Hamburger himself (Ztschr. f. physical Chem., Leipzig, 1890, Bd. vi. S. 319) 

 maintained that permeability did not affect his method, since, by a "vital act" 

 (" Lebenserscheinung"), S. 331, the corpuscles give up to the solution from their juice 

 an amount of some other substance exactly equivalent to that which penetrates from 

 without, so that the total osmotic pressure of the juice is unaltered ! 



4 Hedin, Slcandm. Arch. f. Physiol., Leipzig, 1890 ; Gaertner, Berl. klin. Wchnschr., 

 1892, Bd. xxix. S. 36 ; Koeppe, Arch. f. Physiol., Leipzig, 1895, S. 154 ; Arch. /. d. ges. 

 Physiol., Bonn, 1896, Bd. Ixii. S. 567 ; Miinchen. med. Wchnschr., 1893. No. 24. 



5 Ztschr. f. physikal. Chem., Leipzig, 1891, Bd. vii. S. 529. 



