LIFE AND ENERGY 19 



lost by going into the ball made no perceptible difference in the 

 concentration of this solution. Now, imagine the ball placed in 

 a solution of twice the strength of that within it. The opposite 

 process will take place. Water will pass outwards until the strength 

 of the solution inside has risen to that of the solution outside. 

 The ball will shrink to half its size. We see, then, that such a 

 system behaves exactly like the living cell. But, it may be said, 

 the red blood corpuscles do not contain a solution of cane sugar, 

 True, but the above considerations require only that whatever 

 molecules there are in the solute should be unable to pass through 

 the membrane, no matter what may be the chemical nature of 

 these molecules. The effect is simply proportional to their number 

 in a given volume ; in other words, to the molecular concentration 

 of the solution. 



The movements of water from one side of a membrane to the 

 other side, when caused by difference of molecular concentration, 

 are known as " osmosis'' A membrane which is permeable to the 

 solvent, but impermeable to any particular solute, is called "semi- 

 permeable" as regards that solute. The last name is not very 

 descriptive, but is used in the sense indicated. An "impermeable " 

 membrane would be one which does not permit either water or 

 solute to pass through, such as one made of glass would be. 



We must next devote a little time to the conception of equi- 

 molecular solutions. It is obvious that for chemical operations it is 

 a great convenience to have solutions of which equal volumes 

 contain a known relative number of molecules. For example, 

 suppose that we want to precipitate a solution of sodium chloride 

 by one of silver nitrate. If the solutions are of equimolecular 

 strength, all that we have to do is to take equal volumes, without 

 the necessity of trial ; and if we find that a known volume of the 

 silver nitrate solution is just able to precipitate a particular volume 

 of the sodium chloride solution, we know that these volumes 

 contain an equal number of molecules of the reagents. In practice, 

 the most useful concentrations to take are those in which one litre 

 contains the molecular weight of the solute expressed in grams, or 

 solutions of simple relation to these. The molecular weight of a 

 substance expressed in grams is called a " mol," and hence solutions 

 containing one mol in the litre are " molar" Since the molecular 

 weight of cane sugar is 342, a solution containing 342 gm. in a litre 

 is a molar solution. A molar solution of sodium chloride contains 

 58.5 gm. in the litre, and so on. 



The solution of cane sugar which we have been using contains 

 loo gm. in the litre, a.nd is, therefore, 100/342, or almost exactly 

 0.3 molar. If the red corpuscles behave as osmotic systems, 

 therefore, a solution of glucose of the same molecular concentration 



