POROSITY. 35 



water, is poured upon it, care being taken to prevent a mixing of the 

 two liquids, so as to form two distinct layers, it will be found that 

 after a certain lapse of time the two liquids have mixed with each 

 other, particles of water having entered the alcohol and particles of 

 alcohol the water, until a uniform mixture of the two liquids has 

 taken place. Upon repeating the experiment with a layer of water 

 over a column of solution of common salt, it will again be found that 

 the two liquids gradually enter one into the other until a uniform 

 salt solution has been formed. 



In a similar manner, two or more gases introduced into a vessel or 

 a room will readily mix with each other. /This gradual passage of 

 one liquid into another, of a dissolved substance into another liquid, 

 or of one gas into another gas, ^is called diffusion^ 



Osmose. Dialysis. This diffusion takes place also when two 

 liquids are separated by a porous diaphragm, such as bladder or 

 parchment paper, and it is then called osmose or dialysis. 



The apparatus used for dialysis is called a dialyzer (Fig. 4), and 

 consists usually of a glass cylinder, open at one end and closed at the 

 other by the membrane to be used as the separating medium. This 

 vessel is placed into another, and 

 the two liquids are introduced into 

 the two vessels. If the inner 

 vessel be filled Avith a salt solution 

 and the outer one with pure water, 

 it will be found that part of the 

 salt solution passes through the 

 membrane into the water, whilst 

 at the same time water passes 

 over to the salt solution. 



On subjecting different sub- 

 stances to this process of dialysis, it has been found that some sub- 

 stances pass through the membrane with much greater facility or in 

 larger quantities than others, and that some do not pass through at 

 all. As a general rule, crystallizable substances pass through more 

 freely than amorphous substances. /Those substances which do not 

 pass through membranes in the process of dialysis are known as col- 

 loids, those which diffuse rapidly crystalloids.\ 



Capillary attraction, or, more generally speaking, surface-attraction, 

 is undoubtedly to some extent the cause of the phenomena of osmose, 

 the surface of the diaphragm exercising an attraction upon the liquids. 



