40 CHEMICAL PHYSICS. 



pressure, and counteracted by high temperature and removal of 

 pressure. Thus : One volume of water absorbs at ordinary tempera- 

 ture and pressure about 0.03 volume of oxygen, 1 volume of carbon 

 dioxide, 30 volumes of sulphur 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 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. 



The rate of diffusion is different for different substances. Saline substances 

 may be divided into a number of equidiffusive groups. The quantity of a sub- 

 stance diffused varies also with the temperature. Thus, the rate of diffusion 

 of hydrochloric acid at 49 C. is 2.18 times that at 15 C. The following table 

 gives the approximate times of equal diffusion for the substances named : 



Hydrochloric acid 1.0 Magnesium sulphate .... 7.0 



Sodium chloride 2.3 Albumin 49.0 



Sugar 7.0 Caramel 98.0 



Magnesium sulphate is one of the least diffusible saline substances, yet it 

 diffuses 7 times faster than albumin and 14 times faster than caramel. 



Diffusion is exhibited also by solids, even at ordinary temperature. Sir "\V. 

 Roberts- Austen placed gold and lead in contact four years at 18 C., and 

 found the surfaces united. Gold had penetrated the lead to more than o milli- 

 meters from the surface, while within 0.75 millimeter from the surface gold 

 was found at the rate of 1 oz. 6 pwt. per ton, which would be profitable to 

 extract. 



The phenomena of diffusion can be explained only on the theory that matter 

 is made up of particles or molecules in motion. They are one of the strong 

 links in the chain of evidence upon which the molecular theory of matter is 

 founded. 



Osmose. Dialysis. This diffusion takes place also when two 

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

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



