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 havea 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 er estas 
greater than the escape of syrup, and czss or 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- 
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