52 PRINCIPLES OF GENERAL PHYSIOLOGY 



being filled with the heavier liquid, and having its orifice immersed in the 

 lighter one. Of course, proper correction must be made for the effective 

 weight of the drops in the liquid, compared with that in air. 



Since the surface energy at the contact of two liquids is less than the sum of that betweon 

 each of them and air, it follows that when two liquids, previously in contact with air, are 

 brought into contact with each other, work is obtained. An important fact found by Hardy 

 (1913) is that this work is greatest in the case of the most chemically active fluids, such as 

 esters, alcohols, and acids ; smallest in the case of the saturated hydrocarbons. The merest 

 trace of oleic acid, added to an inactive hydrocarbon, reduces its surface energy to an 

 enormous extent. 



Interfaces between liquids and between these and solids are met with in 

 physiology more frequently than those between gases and liquids. 



As regards the surface tension at the interface between solid and liquid, we 

 have, unfortunately, no direct method of determination, but Ostwald (1900, ii. 

 p. 503) indicated an indirect one, depending on the greater solubility of small 

 particles than of large ones. This fact is due to the action of molecular forces 

 at the interface, causing the liquid component to have greater solvent power. 

 The larger the total area of surface on the particles, the greater will their 

 solubility appear to be. This is the reason why large crystals grow at the 

 expense of small ones, since the solution which is saturated as regards the 

 large particles or crystals is not saturated with respect to the smaller ones 

 (see also the book by Freundlich, 1909, pp. 143-145). 



W. J. Jones (1913) has made renewed measurements by the method referred 

 to, and finds that the surface tension of barium sulphate, in contact with its 

 saturated solution, is 1,300 dynes per centimetre. It. will be noted that this is a 

 very high value compared with that between liquid and liquid, or liquid and 

 gas. At the water -air interface, for example, the surface tension is only 

 75 dynes. The fact is of importance in connection with he large degree of 

 adsorption manifested by the surfaces of solids, such as charcoal, as will be 

 seen later. 



As a rule, substances in solution in liquids lower the surface tension at the 

 interface between these liquids and air. Inorganic salts (such as sodium 

 chloride) raise it, but not to any great extent. There are great differences 

 between the actions of different substances in their action on surface tension. 

 Home, bile salts, for example, have a very great effect. The same statement 

 applies to the interface between liquid and liquid, except that it appears that 

 all bodies in solution, even inorganic salts, lower the surface tension. 



W. C. M'C. Lewis (1909, 1, p. 469) finds that inorganic salts lower the interfacial tension 

 between a hydrocarbon oil and water. He also calls attention (1910, 1, p. 632) to the 

 circumstance that if we take into account the curvature of the surface, and the densities of 

 the two phases, we obtain a quantity, which may be called the "specific capillary constant," 

 and that this constant is always lowered by dissolved substances, even when air is one of 

 the phases. 



A point to be remembered is that small amounts of dissolved substances 

 produce, for equal amounts, a greater lowering of surface tension than larger 

 amounts. The curve expressing the relationship is one of the family of 

 parabolas (Freundlich, 1909, p. 65). The importance of this will be seen when 

 we are discussing adsorption. 



When living protoplasm is in contact with any solution, there must be surface 

 tension at the interface. Some experiments by Kisch ( 1 912, p. 152) are of interest 

 here. Yeast and other fungi were found to be permanently injured as soon as the 

 surface tension of the solution in which they were immersed became, by the 

 addition of various substances, less than half of that between water and air. The 

 actual concentrations required were : 



Ethyl alcohol - 28 per cent. 



Isoamyl alcohol - 2 



Acetone - 30 



The cells of higher plants were found by Czapek ("Ueber eine Methode zur 

 direkten Bestimmung der Oberfliichenspannung der Plasmahaut von Pflanzen- 



