FUNDAMENTAL PROPERTIES OF MATTER. 41 



The apparatus used for dialysis is called a dialyzer (Fig. 1 5), 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 with 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-attrac- 

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

 osmose, the surface of the diaphragm exercising an attraction upon 

 the liquids. 



Diffusion through the membrane will not take place unless the 

 membrane is in contact with water, and, moreover, its limit will be 

 reached when the concentration outside is the same as that inside the 

 dialyzer. Hence, a large quantity of water should be on the outside 

 and often renewed. Generally, water flows through the membrane 

 toward the denser liquid, which increases in volume, but alcohol and 

 ether are exceptions. They act like liquids which are denser than 

 water. In the case of acids, water flows either into the acids or from 

 the acids, according as they are more or less dilute. When a dilute 

 alcohol is kept for a time in a bladder the volume diminishes, but 

 the alcoholic strength increases. The reason of this is, no doubt, 

 that the bladder permits the water to pass rather than the alcohol. 



An interesting effect of osmosis is seen in the purgative action of 

 magnesium sulphate. A solution of this salt is not readily absorbed 

 and causes a flow of water from the intestinal bloodvessels by 

 osmosis, which, together with the direct stimulating action of the 

 salt to peristalsis, causes purging action. All strong saline solutions, 



