THE CELL AN OSMOTIC APPARATUS. 



229 



In certain experiments by Pfeffer, made with a single cell, in 

 which was a solution of cane-sugar containing a trace of one of 

 the membranogenic substances, while the water outside contained 

 a trace of the other, the following pressures were indicated : 



With a 3.3 per cent solution of potassic nitrate in the cell, 

 Pfeffer obtained a mercurial pressure of 436.8 cm. 



616. An active vegetable cell is an osmotic apparatus. The 

 chief agent in its work of absorption is the peripheral film of 

 the colloidal protoplasmic mass, and this receives mechanical 

 support from the wall of cellulose in which it is held. It was 

 formerly believed that in osmosis there is always an exchange 

 of materials, one current passing inwards (endosmose), the 

 other outwards (exosmose) ; and there are numerous cases in 

 which this is true, and in which the osmotic equivalent can be 

 calculated (see 612). But Pfeffer's experiments show how great 

 a force may be exerted by osmosis in cases in which there is 

 little or no substance passing out to replace the liquid ab- 

 sorbed. In the series of experiments in which a solution of 

 sugar was employed in his osmotic apparatus, no trace of this 



properly done, any small quantity of air which may be in the upper part of the 

 cork will rise during insertion to the capillary point. Gradually and cautiously 

 warm the tube, beginning close to the cork. This will expand the fluid and 

 drive the air wholly out. At the moment when the solution completely fills 

 the tube, fuse the capillary point in the spirit-lamp. The cell is now entirely 

 free from air and hermetically sealed. During the time of inserting the ma- 

 nometer, corking, and sealing, the porcelain part of the cell must not be allowed 

 to become dry, but must be frequently dipped into the solution from which it 

 was taken. With unannealed brass wire secure both corks after the fashion 

 of champagne-bottles. 



Now suspend the cell in the solution of copper sulphate so that the porce- 

 lain shall be wholly submerged but shall not touch the sides of the vessel 

 containing the solution. Xote the position of the mercury in the manometer, 

 and see that the temperature remains constant in the room. If the cell is 

 perfect, a certain degree of pressure will be indicated in less than an hour. 



