34 INTRODUCTION. 



of oxygen, 1 volume of carbon dioxide, 30 volumes of sul- 

 phur dioxide, and 800 volumes of ammonia. 



Diffusion. When a cylindrical glass vessel has been partially 

 filled with water, and alcohol, which is specifically lighter than 

 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 mix- 

 ture of the two liquids has taken place. Upon repeating the 

 experiment with a layer of water over a column of a 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. 



Endosmosis. Dialysis. This diffusion takes place also when two 

 liquids are separated by some porous diaphragm, such as bladder 

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

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



usually consists of a glass cylin- 

 der, open at one end and closed 

 at the other by the membrane 

 to be used as the separating 

 medium. This vessel is then 

 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 



Dialyzer. . 



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 substances to this process of dialysis, 

 it has been found that some substances pass through the mem- 



