38 CHEMICAL PHYSICS. 



depressions, all other circumstances being equal, are inversely pro- 

 portional to the diameters of the tubes. 



Defining the phenomena of capillary attraction more scientifically, 

 we may say that the adhesive force of glass, wood, etc., for water and 

 most other liquids exceeds the cohesive force acting between the 

 molecules of these liquids, while in mercury the cohesive force pre- 

 dominates over the adhesive. 



The rise or fall of liquids in capillary tubes is explained thus : There is an 

 attraction between the particles in the surface of a liquid which causes the 

 surface to act like a stretched membrane. It requires force to separate the 

 particles in the surface and this force is spoken of as the surface tension of the 

 liquid. Proof of this tension is seen in the fact that insects can stand on water 

 without breaking through, and that an oily needle can float on water although 

 it is heavier than it. Another principle in physics is that a stretched surface 

 of a spherical form exerts a force toward the center of curvature of the surface, 

 and tends to contract toward the center. Illustrations of this are seen in the 

 fact that drops of liquids assume a spherical form, and that soap bubbles con- 

 tract when the air in the interior of them is allowed to escape. Now, when a 

 fine tube is dipped into a liquid that wets it, particles of the liquid are drawn 

 up by adhesion of the glass, thus causing a curved surface, which, acting like a 

 membrane, draws up the particles below it by cohesion. The liquid rises until 

 the weight of the column counterbalances the cohesion between the particles 

 in the curved surface, that is, the surface tension. Thus we see also why liquids 

 rise higher in fine tubes than wider ones, since it requires a shorter column 

 of liquid of greater diameter to equal in weight a longer column of smaller 

 diameter. Moreover, since surface tension of liquids diminishes as tempera- 

 ture rises, we see why capillary elevation diminishes as temperature increases. 

 The same kind of argument will explain why liquids which do not wet a tube, 

 for example, mercury, will sink in. the tube below the level of the liquid 

 outside. 



Familiar examples of capillary phenomena are the action of lamp-wicks, 

 the rise of water in wood, sponges, bibulous paper, sand, sugar, and the rise of 

 sap in the vessels of plants. 



Surface-attraction of solids for gases. Any dry solid sub- 

 stance, carefully weighed, will, after having been exposed to a higher 

 temperature, show a decrease in weight while yet warm. Upon 

 cooling, the original weight will be restored. This fact cannot be 

 explained otherwise than that some substance or substances must 

 have been expelled by heat, and that this substance or these sub- 

 stances are reabsorbed on cooling. 



This is actually the case, and the substances expelled and reab- 

 sorbed are the gaseous constituents of the atmospheric air, chiefly the 

 aqueous vapor. 



Every solid substance upon our earth condenses upon its surface 



