248 PHYSIOLOGICAL PHYSICS. ichap. xxn. 



on its surface in drops. Similarly, a membrane 

 which contains water in its pores, on being placed in 

 alcohol, expels a considerable quantity of water, 

 because it can contain of alcohol only about one- 

 seventh of what it can contain of water. As a result 

 of this expulsion of water, the texture shrinks. 



Diffusion of liquids. Different liquids exercise 

 attractive forces between their molecules, just as the 

 molecules of a solid body and those of a liquid 

 coming into contact develop attractive forces. 

 When one liquid is in contact with another, if the 

 force of attraction exercised between the mole- 

 cules of the one liquid and those of the other are 

 greater than the forces of cohesion exercised between 

 the molecules of each liquid separately, then the two 

 liquids will be capable of advancing into one another's 

 substance, that is, will be miscible. If, however, the 

 forces of cohesion between the molecules of one liquid 

 are sufficient, they are superior to the force of attrac- 

 tion exerted by the other liquid ; and the liquids 

 remain separate and independent. They are not 

 miscible. Of this nature are water and oil, and water 

 and mercury. When the different liquids, then, whose 

 molecules mutually attract one another, are placed in 

 contact with one another, they proceed to mix, and 

 in time the mixture will become uniform. This is 

 called DIFFUSION. 



A similar thing occurs when a liquid dissolves a 

 solid body with which it is in contact. The liquid over- 

 comes the force of cohesion between the molecules of 

 the solid body, separates them, and the two then form 

 a homogeneous liquid. A point is reached when the 

 liquid is unable to overcome any more the cohesion, 

 between the molecules of further quantities of the 

 foreign body. In this case the point of saturation of 

 the liquid is reached. This point of saturation varies 

 with the solid body, and the liquid which dissolves it. 



