LIFE AND ENERGY 17 



sugar which the cells contain can also be shown by chemical tests 

 not to be washed out by water, as long as the cell is normal. 



At the interfaces between different phases inside the cell, 

 membranes must also be formed, and it is easy to see their 

 importance in keeping separate the various reactions going on 

 within a cell at the same time. The difficulty of finding out what 

 is happening in the space of a single cell is very great, and we do 

 not yet know much about it. 



But, it may be said, supposing that the membrane is like a sieve, 

 with holes through which such small molecules as those of water can 

 pass, but which are too small for large molecules like the aniline 

 dyes, and there are many reasons for believing that such is their 

 structure (P., pp. 113, 114), how does it behave to molecules which, 

 although comparatively small, are larger than those of water, say 

 sodium chloride? The greater number of these substances of 

 physiological importance are colourless, so that some indirect way 

 of testing the permeability of the cell membrane to them must be 

 made use of. We have seen that the membrane is impermeable to 

 cane sugar, and we need to test it as regards glucose and sodium 

 chloride especially. 



Osmosis 



The most convenient way of doing so is by taking advantage 

 of the phenomena of '' osmosis " and their consequences. Here we 

 come upon a property of solutions that is of some difficulty to 

 explain and to understand. The reader may be reminded that 

 there are different ways of looking at it, but that given below is 

 probably the most intelligible to begin with. 



Let us first make a few simple experiments to see what happens 

 to red blood corpuscles when placed in water and various other 

 solutions (E., p. 169). We take these bodies as convenient repre- 

 sentatives of the cells of the higher animals, especially so for the 

 present purpose since they are not attached together, and can be 

 examined in the uninjured state with ease. Having a thin film of 

 blood under the microscope, note the size of the corpuscles. Run 

 in a 10 per cent, solution of cane sugar. No change will be seen. 

 This being so, we may dilute the blood with such a solution at 

 once, a procedure which will render the observation of separate 

 corpuscles an easier matter. Next, try the effect of a 5 per cent, 

 solution. The corpuscles will swell up and may burst. This 

 occurs so rapidly if water itself be used, that it is difficult to see 

 what has happened. The only possible conclusion to be drawn is 

 that the corpuscles suck up water until they burst. Test, finally, 

 the effect of a stronger solution; the corpuscles will shrink. Similar 



