experiments can be made with the various other kinds of cells 

 making up the bodies of animals, but they require rather more 

 indirect methods. It has been found that the cells of warm-blooded 

 animals remain of a normal size in solutions of cane sugar only 

 when it is ab6ut 10 per cent, the exact strength differing slightly 

 in the various species. The cells of the frog or fish require a 

 solution of less strength. 



What is the explanation of this behaviour? 



Suppose that we have a small hollow ball made of an elastic 

 material, which has minute pores in it large enough to allow the 

 molecules of water to pass through, but too small for those of cane 

 sugar to pass. This is filled with a 10 per cent, solution of sugar, and 

 immersed in water. It would swell up rapidly, and ultimately burst. 

 The fact which has to be explained is the rushing in of water 

 molecules at a greater rate than they escape, although the membrane 

 is completely permeable to them in both directions. It is somehow 

 due to the presence of cane sugar molecules on the inside of the 

 membrane and their absence on the outside, because this is the 

 only difference. But how? We call to mind the fact that molecules 

 have an actual size and that, in a cane sugar solution, a part of the 

 space is taken up by the solute and, therefore, there are fewer water 

 molecules than in an equal volume of water. Giving our attention 

 next to a particular area of the membrane, we realise that on the 

 outside the whole space is bombarded by water molecules, so that 

 wherever there is a pore, a water molecule can get through. On 

 the inside, a number of these pores will be hit by sugar molecules, 

 which cannot get through. As concerns those hit from the inside 

 and outside by water molecules, as many will pass in a given time 

 in both directions, since the space is merely a part of the general 

 mass of water. But where the sugar molecules hit, no \vater passes 

 outwards, while there is no hindrance to its passing inwards. The 

 amount that enters is, therefore, proportional to the number of sugar 

 molecules in a given volume of the solution. 



If we immerse the ball in a solution of cane sugar of the same 

 strength as that inside it, the number of pores hit by sugar molecules 

 is the same on both sides, so that there is the same limited oppor- 

 tunity for water to pass inwards and outwards, and no change 

 takes place in the quantity of water within. If we place the ball 

 in a solution of half the strength of that inside it, what will happen? 

 Water will enter, because there are more pores free on the outside 

 than on the inside. But, as the water enters, the solution becomes 

 diluted. The ball will expand until its volume has become double 

 that which it first possessed ; since, then, the solution within will 

 have become of the same strength as the outer solution, supposing 

 that we had a large volume of solution outside, so that the water 



