LIFE AND ENERGY 21 



Osmotic Pressure 



Hitherto, we have considered the effect of the entrance of water 

 in producing a swelling of cells. Suppose that they cannot swell, as 

 is the case with the cells of the higher plants, encased in a cellulose 

 box. What will happen ? We may imagine that the membrane of 

 our original ball is rigid and incapable of being stretched, and 

 that we have attached a vertical tube to it, so that the water which 

 enters in may find an outlet. If 10 per cent, cane sugar be inside 

 and water outside, we shall see that the solution rises rapidly in the 

 vertical tube, and finally runs over the top. A pressure is evidently 

 produced by the inflow of water, and this pressure must be greater 

 than that of the column of liquid of the height of the tube. This 

 would, indeed, be expected when we call to mind that the molecules 

 of water get in from the outside in virtue of their kinetic energy. 

 The amount of energy inside the ball is clearly greater than before 

 the extra molecules of water had entered. The increase of pressure, 

 which shows itself by raising the column of liquid in the tube, is 

 what is called the " osmotic pressure " of the solution. It is difficult 

 to make a simple experiment to show this fact, because, although 

 artificial membranes can be made which are semi-permeable as 

 regards cane sugar, it is not an easy matter. But there are some 

 organic substances whose molecules are large enough not to pass 

 through the pores of parchment paper, which are much larger than 

 those of the cell membrane. The experiment may be tried with 

 gum arabic, or with the protein of milk, called caseinogen (E., p. 171). 



We must next get an idea of how great osmotic pressure is. 

 Returning to our membrane impermeable to cane sugar, let us try 

 a much more dilute solution, and instead of allowing it to raise 

 a column of itself in a tube, let it raise the heavier mercury, as can 

 easily be done by connecting the ball to a mercury gauge or 

 " manometer." Pressures of moderate degree are usually expressed 

 in millimeters of mercury, 760 mm. being the pressure of the 

 atmosphere. Taking a I per cent, solution, that is, 0.03 m., we 

 should find that the mercury rose to a height of 511 mm., and 

 if we took other concentrations, we should find that the pressure 

 was very nearly in proportion to the concentration, so that we may 

 say that the osmotic pressure of the red blood corpuscles and the 

 contents of other animal cells is about 5,110 mm. of mercury, 

 or 6.7 atmospheres. The osmotic pressure of cane sugar solutions 

 has been very accurately measured, and it has been found that the 

 volume taken up by the molecules and other connected phenomena 

 have to be taken account of. They naturally play a much larger 

 part when the solutions are concentrated. 



The osmotic pressure of the cell contents is a high one, even 



