[From the Encyclopaedia Britannica.] 



LXXXIX. Diffusion. 



SOME liquids, such as mercury and water, when placed in contact with 

 each other do not mix at all, but the surface of separation remains distinct, 

 and exhibits the phenomena described under CAPILLARY ACTION. Other pairs 

 of liquids, such as chloroform and water, mix, but only in certain proportions. 

 The chloroform takes up a little water, and the water a little chloroform; 

 but the two mixed liquids will not mix with each other, but remain in con- 

 tact separated by a surface shewing capillary phenomena. The two liquids are 

 then in a state of equilibrium with each other. The conditions of the equi- 

 librium of heterogeneous substances have been investigated by Professor J. 

 Willard Gibbs in a series of papers published in the Transactions of the Con- 

 necticut Academy of Arts and Sciences, Vol. in. part I. p. 108. Other pairs 

 of liquids, and all gases, mix in all proportions. 



When two fluids are capable of being mixed, they cannot remain in equi- 

 librium with each other; if they are placed in contact with each other the 

 process of mixture begins of itself, and goes on till the state of equilibrium 

 is attained, which, in the case of fluids which mix in all proportions, is a 

 state of uniform mixture. 



This process of mixture is called diffusion. It may be easily observed by 

 taking a glass jar half full of water and pouring a strong solution of a 

 coloured salt, such as sulphate of copper, through a long-stemmed funnel, so 

 as to occupy the lower part of the jar. If the jar is not disturbed we may 

 trace the process of diffusion for weeks, months, or years, by the gradual rise 

 of the colour into the upper part of the jar, and the weakening of the colour 

 in the lower part. 



This, however, is not a method capable of giving accurate measurements 

 of the composition of the liquid at difierent depths in the vessel. For more 



VOL. ii. 79 



