OSMOSIS. 271 
freezing-point method, il is found to be higher (about 1*89). Other 
indices of "isoton* " than the plasmolysis of the vegetable cell have 
been used by physiolog 
Hamburger 1 has used red blood corpuscles. If these are placed 
in solutions of substances which do uot 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 substanc -. 
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 eentrifugalised 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, 1S36, Phys. Abth., S. 476; 1887, S. 31; 
Ztschr. f. physikal. Chan.. Leipzig, 1890, Bd. vi. S. 319. 
- Arch. f. d. ges. Physiol., Bonn, 1896, Bd. lxiii. S. 86. 
8 Hamburger himself [Ztschr. f. physikal. Chen., Leipzig, 1890, Bd. vi. S. 319) 
maintained that permeability did not affect his method, since, by a "vital act" 
(" Lebeuserscheinung"). 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, SJcandin. Arch. f. Physiol., Leipzig, 1890 ; Gaertner, Berl. Idin. Wchnschr., 
1892, Bd. xxix. S. 36 ; Koeppe, Arch. f. Physiol., Leipzig, 1S95, S. 154 ; Arch. f. d. ges. 
Physiol., Bonn, 1896, Bd. lxii. S. 567 ; Munchen. med. Wchnschr., 1893. No. 24. 
5 Ztschr. f. physikal. Chem., Leipzig, 1891, Bd. vii. S. 529. 
