THE MEASUREMENT OF OSMOTIC 

 PRESSURE BY DIRECT EXPERIMENT 



By T. MARTIN LOWRY, D.Sc. 



A. Osmosis and Osmotic Pressure 



As long ago as 1748 it was discovered by Nollet that a flow of 

 water took place through a membrane of pig's-bladder separating 

 alcohol from water. This observation was forgotten during 

 more than half a century, until it was redescribed in 1802 by 

 Parrot, 1 who also detected a similar flow when urine was used 

 instead of alcohol. Parrot recognised that a flow of liquid took 

 place simultaneously in both directions but that the velocities 

 differed so widely that a pressure might be developed, on one 

 side of the membrane, equivalent in some cases to a column 

 of water not less than 10 ft. in height. Quantitative measure- 

 ments made by Dutrochet (1827), to whom we owe the terms 

 exosmose and endosmose and by Vierordt (1848) showed that 

 the rate of flow depended on the nature of the membrane, on the 

 concentration of the solution and on the temperature; but 

 the factors determining the flow were too complex to allow of 

 any simple statements of the laws governing osmosis. One 

 of the first generalisations to be attempted was suggested by 

 Jolly in 1848, when he brought forward evidence to show that 

 a fixed ratio existed between the exosmosis or outward flow 

 of the salt through the membrane and the endosmosis or inward 

 flow of water into the solution. This ratio, the "endosmotic 

 equivalent," he supposed to be independent of the concentration 

 but further investigation showed that this was not the case. 



Equally little progress was made when experiments were 

 carried out to determine the maximum " head " of liquid which 

 could be driven up by the osmotic flow of water into a solution. 

 It is true that one factor, the frictional resistance of the 

 membrane to the endosmotic flow, was now eliminated ; but 

 so long as an exosmotic flow still took place the "head" of 



1 See Walden, "Die Hauptdaten aus der Geschichte des Osmotischen Drucks 

 und der Osmotischen Losungstheorie," Bull Acad. Sa\, St. Petersburg, 1912. 



544 



