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CHAPTER VI 



OBSERVATIONS WHICH LEAD TO THE THEORY THAT ALL MATTER 

 IS MADE UP OF VERY SMALL SEPARATE PARTICLES 



62, When Liquids differing in Composition are placed in 

 Contact a gradual Rearrangement of the Matter may proceed 

 until the whole is homogeneous, i.e. until the Composition is 

 the same in every Part. The change by which two liquids 

 spread themselves through one another so that in course of 

 time the whole is alike in composition, is called diffusion. It 

 will be found to take place in gases as well as liquids. Some 

 liquids, however, remain distinct, and do not diffuse even when 

 left in contact for a very long time. Such cases are illus- 

 trated by mercury and water. Mercury added to water sinks 

 below the water, and their surface of separation does not 

 change. In other cases, however, the surface of separation 

 disappears more or less rapidly, the different kinds of matter 

 cross the original surface of separation in each direction, and 

 gradually become evenly distributed. 



When a liquid and a gas are in contact the same kind of 

 change may go on. For example, when water and air are in 

 contact, some of the water passes into the air and some of 

 the air into the water ; but the substances differ so much in 

 condition that the laws which hold for substances in the 

 same condition do not apply. The solution of gases in liquids, 

 and the evaporation of liquids, are treated separately. But 

 within certain limits we may consider that if we have any 

 two kinds of matter, x and y, in contact there is a tendency 

 for the original surface of separation to become divided into 

 two surfaces, which ultimately diverge as completely as 



