RELATION OF WATER TO THE PROTOPLASM 55 



nature of the process by which water makes its entry into 

 a cell. This is based upon a physical process which is 

 known as osmosis. 



When two fluids of different densities, such as water and 

 syrup, are separated from each other by a homogeneous 

 permeable membrane, they will tend to pass through the 

 latter in both directions till there is a mixture of the two of 

 equal density on each side of it. We shall thus have a stream 

 of water passing through the membrane to the syrup, and a 

 stream of syrup similarly passing to the water. The rate 

 of flow of the two streams will not be the same, however, 

 and the first result will be a considerable increase of the 

 volume of the liquid upon the side of 

 the membrane in contact with the syrup, 

 owing to the greater amount of water 

 that will have passed through. 



A convenient form of apparatus to 

 exhibit this process of osmosis is repre- 

 sented in fig. 50. It consists of a 

 bladder fastened to the end of a narrow 

 tube which is immersed, as shown, in a 

 vessel of water. The bladder and part 

 of the tube are filled with syrup, and the 

 height at which the latter stands in the 

 tube is noted. After some time the 

 contents of the tube will be increased in 

 consequence of the entry of water being FlG - so. APPARATUS 



,. TO SHOW THE PRO- 



greater than the escape of syrup, and CESS OF OSMOSIS. 



the liquid will stand at a higher level 



in the tube. If the positions of the water and the syrup 



had been reversed, the liquid would have fallen in the tube, 



showing that the greater osmotic stream was in the opposite 



direction. 



The relative difference in the rate of the two streams 

 will vary with the concentration of the syrup. 



Other substances than sugar have a similar power of 

 setting up osmotic currents, which indeed is especially pro- 



