OSMOTIC PRESSURE 



39 



A similar determination of isotony by physiological 

 means, in this case, however, taken from the animal 

 organism, may, according to Hamburger 1 , be carried out 

 with red blood corpuscles, the solution to be studied being 

 shaken up in a test tube with a couple of drops of defi- 

 brinized blood. Two phenomena are then observed, 

 according as the solution exercises a great or small osmotic 

 pressure : in the first case the corpuscles give up their 

 colouring matter to the solution ; in the second they sink 

 to the bottom of the colourless liquid. Liquids which lie 

 on the margin in this respect are isotonic, and so we have 

 an easily available means for molecular weight determina- 

 tions. 



Similar isotony has been observed by Tammann 2 without 

 physiological aid, by means of a precipitated membrane, 

 that is a colloidal skin formed by contact of two solutions. 

 The most available material yet found for this purpose is 

 copper ferrocyanide, obtained by contact of potassium 

 f errocyanide and copper sulphate in layers ; this was used, 

 and the passage of water in one or the other direction 

 observed by means of Topler's interference apparatus: 

 where water enters, a rising stream in the specifically 

 heavier salt solution is observed ; where the reverse 

 occurs, a descending stream due to local increase in 

 density. 



(6) Absolute Measurement of Osmotic Pressure. As 

 already mentioned, absolute pressure measurements of 

 osmotic phenomena are rendered difficult by the fact 

 that it is not only necessary to prepare a semipermeable 

 partition, but one that has sufficient resistance to stand 

 the commonly great mechanical pressures involved. The 

 organic membranes, which were found so well adapted for 

 determining isotony, are for that reason useless for the 

 present purpose, and consequently there are only a few 

 isolated experiments that have been carried out with success, 



1 Zeitschr.f. Phys. Chem. 6. 319. 



2 Wied. Ann. 34. 299. 



