[macallum] cellular OSMOSIS AND HEREDITY 147 



These films are, however, so delicate and so easily iniptured, that 

 they cannot be niade to furnish data which would enabie us to detennine 

 the pressure exercised by the dissolved substances, PfefPer ^ met the 

 difficulty which they present by causing the precipitate of cupric fer- 

 rocyanide to be deposited in the walls of imglazod porous earthenware 

 or porcelain vessels. This he succeeded in doing by pouring one of the 

 solutions, e.g., the cupric sulphate, in the porous vessel which was then 

 placed in the other solution. The two solutions diffused through the 

 poiies of the wall of tJie vessel and met forming' a precipitate film wihich, 

 owing to the firm support afforded by the wall, could not be displaced, 

 and, after washing away and dissolving out all traces of the two solutions, 

 the porous pot was now made to serve as a membrane to detemiine the 

 pressure exercised by dissolved substances. This was done by putting into 

 the porous receptacle some of the solution which required to be examined, 

 closing the vessel with a firmly fitting stopper through which penetrated 

 a manometer connection to indicate the pressure. When the vessel was 

 placed in distilled water the latter passed through the wall of the por- 

 celain tube and as there was apparently no corresponding movement in 

 the opposite direction, the pressure inside rose and was registered ])y the 

 r^ianometer. 



With such membranes, Pfeffer obtained some striking results, but 

 the chief one was that the pressure was foimd to be dependent on the 

 strength of the solution, bedng almost proportional to the concentration 

 in ihe case of organic solutés like cane sugar and dextrose and alsol in 

 proportion to the rise of absolute temperature, but in the case of salts 

 the results were not so constant. 



Pfeffer did not account for the causation of his results, and it was 

 van't Hoff who wias the first to offer an explanation of them. This ex- 

 planation practically embodies the now well known theory of the gas 

 nature of solutions which is accepted widely as a cardinal principle of 

 physical chemistry. 



Pfeffer and Traube, in postulating the existence of a semi-permeable 

 membrane led by a desire to explain how it happens that the living cell 

 and particularly, the vegetable cell, placed in aqueous media maintains its 

 normal turgor and at the same time netains all its organic constiituenbs, 

 dextrose for example. The facts then known apparently indicated the 

 existence > of a membrane which allowed water to enter the cell, but not 

 the organic substances to escape. Such a membrane implied a semi- 

 permeable character. 



* Osmotische Untersuchungen. Leipzig. 1877. 



