4 2 4 ABSORPTION 



possible, just as if it were a stretched elastic membrane. The cause 

 of this tension is to be sought in the mutual attraction exerted by mole- 

 cules which are very close to each other. This molecular pull is 

 enormously strong. It has been calculated, for instance, that the 

 so-called internal pressure which is due to it is in the case of water not 

 less than 23,000 atmospheres. In the interior of the drop each mole- 

 cule, being surrounded by other molecules, is pulled by this attractive 

 force equally in all directions that is to say, on the whole it is not 

 pulled at all, since the pulls of all the surrounding molecules balance 

 each other. At the free surface, on the contrary, the molecules are 

 pulled towards the surface, but not away from it, and the pull of the 

 molecules below the surface layer is not balanced by the pull of mole- 

 cules above it. The resultant tension on this layer is the surface tension. 

 Changes in the amount of this surface tension in the case of a given 

 liquid can be produced by bringing gases, solids, or other liquids into 

 contact with the surface layer that is, by bringing molecules of other 

 substances so near the surface molecules of the liquid that they can 

 attract them, and so to a greater or less degree, depending upon the 

 nature of the substances, balance the attraction of the molecules 

 beneath the surface of the drop. Another way in which the surface 

 tension can be altered is by changing the temperature. The higher 

 the temperature, the greater is the average velocity with which the 

 molecules of the liquid are moving, and the greater the average distance 

 between the molecules (expressed as the expansion of the liquid) . Increase 

 of temperature therefore causes the molecules, through the kinetic 

 energy of heat imparted to them, say, from an external source, to 

 repel each other, and to that extent counteracts their mutual attrac- 

 tion. Accordingly, at the surface the tension, which, as stated, depends 

 . upon the excess of this attraction acting towards the interior of the 

 drop, will be diminished. When the temperature is diminished the 

 surface tension will increase. The surface tension can also be altered 

 by altering the electrical charge on the surface. An instance of this 

 is described on another page in connection with the capillary electrom- 

 eter (p. 702). In such ways, then, the surface tension at the inter- 

 face where the cells lining the intestine come into contact with the 

 contents of the gut or with the tissue lymph, or at the interfaces within 

 the cells where solid and liquid ' phases ' come into contact with each 

 other or where different liquids touch, may undergo alterations in 

 either direction. If the tension of the surface is altered, the surface 

 energy or power of doing work inherent in the existence of this tension 

 will, of course, be altered too. In this way the energy, or a portion of 

 it, which is unquestionably expended in absorption may be supplied 

 ultimately at the expense of the chemical energy of cell constituents 

 or of food substances on their way through the cells, by means of which 

 the original surface tensions are restored. It has been surmised that 

 changes of surface tension may also be concerned in the secretion of 

 glands, in muscular contraction (p. 718), and other functions (Macallum, 

 Bernstein, etc.). 



Adsorption. Connected with the peculiar properties of surfaces 

 referred to in the last paragraph are certain phenomena spoken of as 

 adsorption phenomena. Adsorption is typically seen when a solid in 

 such a form that the surface is greatly increased (e.g., a fine powder or 

 a colloidal solution in which the substance is suspended in the form 

 of exceedingly small particles) is placed in contact with a gas or a 

 solution. There occurs a diminution in the concentration of the gas 

 or the dissolved substance, and a corresponding accumulation of it on 

 the surface of the solid. Equilibrium is rapidly established, and the 



