ABSORPTION AND MOVEMENT OF WATER 109 



a change in the composition of both. The molecules of 

 salt, being free to move not only through the one volume 

 in which they were dissolved, but also into and through the 

 second volume, will do so, and there will therefore come to 

 be finally an equal distribution of salt molecules in the two 

 volumes, each volume then containing two and one half 

 grammes of salt. To such molecular movement, unaided by 

 stirring, jarring, or other mechanical means, the name diffu- 

 sion is given.* Let us suppose now that one volume of 

 water contained five grammes of common salt and the 

 other five grammes of any other salt, say potassium nitrate. 

 The diffusion of the common salt from the first into the 

 second volume of water, and of the potassium nitrate from 

 the Second into the first volume of water, would be at practi- 

 cally the same rate as into pure water. Theoretically there 

 should be a difference in rate ; actually there is no difference 

 which can be detected. There would presently be two and 

 one-half grammes of common salt and two and one-half 

 grammes of potassium nitrate in each volume of water. 

 But if we had five grammes of common salt in one volume 

 and two grammes of the same salt in the other, the diffu- 

 sion would not be so rapid, although the mixture would 

 finally be as perfect. The rate of diffusion will vary with 

 the difference in the proportions of salt in the two volumes, 

 the greater the difference at the beginning the more rapid 

 the diffusion; the nearer the proportions come to being 

 equal the slower the diffusion, till, ultimately, with equal 

 proportions, the diffusion ceases. So long as there is no 

 chemical action of one salt upon another, this rule applies 

 as well to solutions containing mixtures of salts as to 

 solutions of single salts. 



If now we bring together two equal volumes of pure water, 

 only interposing a permeable membrane ( bladder, vegetable 

 parchment, cell-wall) between them, there will be the same 

 molecular movements as in the first case above supposed, 

 but there can be no change in composition. Similarly there 



* Illustrative experiments on diffusion are described in Darwin and 

 Acton's and in the other laboratory manuals of plant physiology already 

 referred to (p. 27). 



