50 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



molecules of liquids are also capable of exerting an attractive force on 

 solids, or adhesion. In addition to these two modes of manifestation 

 of molecular force, molecules of liquids are capable of attracting mole- 

 cules of other liquids. If in two different liquids brought into contact 

 the cohesive force between the molecules of each liquid is greater than 

 the attractive force between the molecules of the different liquids, the 

 liquids remain separate and apart, and are said not to be miscible. 

 Water and oil furnish examples of such liquids. 



If, however, the attraction between the molecules of the different 

 liquids is greater than the cohesive force of either, then the liquids will 

 mix, even against gravity, until the mixture becomes uniform. Such a 

 process of mixing is called diffusion of liquids. It follows, therefore, 

 that whenever two chemically indifferent fluids are brought into contact 

 with one another the}^ mix, even without any disturbing cause, until a 

 perfectly uniform mixture results. 



Diffusion may be illustrated by filling a small bottle with some saline 

 solution and then placing it in a large jar, which is then carefully filled 

 with distilled water, so poured in as to cover the mouth of the small jar 

 which contains the saline solution, at the same time avoiding any mixing 

 of the fluids by movement. 



If a small portion of the water in the large jar is then drawn off 

 carefully from time to time with a pipette, it will be found that the 

 water will contain a gradually increasing quantity of the salt, until 

 finally the jar will be filled with a perfectly uniform saline solution. 



From such experiments it has been found that the rapidity of dif- 

 fusion increases with the extent of surfaces in contact, with the tempera- 

 ture, and with the difference in concentration of the two fluids ; it is, 

 therefore, more rapid at the beginning of the experiment, when the outer 

 jar contains distilled water, than later, when it contains a saline solution. 

 The rapidity of diffusion also varies with the chemical nature of the 

 solutions ; thus, potassium salts diffuse much more readily than sodium 

 salts, a point which we shall later see is of great importance. Acids dif- 

 fuse very rapidly; alkaline salts and sugar slower, and colloids, perhaps 

 from the fact that they cannot form true solutions, scarcely at all, though 

 colloids in solutions with crystalloids do not interfere with the diffusion 

 of the latter ; while, when two salts undergo diffusion together, the least 

 diffusible salt diffuses more rapidly than it would alone. 



In the different animal tissues, in addition to the intermolecular 

 spaces filled with fluid, we have also larger spaces, or sensible pores, 

 which contain fluids, and which form a system of more or less fine canals 

 traversing the different parts of the body (lymph-spaces, lymph- and 

 blood-capillaries). Therefore every internal part of the animal body is 

 continually bathed in liquid into which fluids leaving the cells are 



