PRESIDENTIAL ADDRESS. 745 



If, on the other hand, an organic compound — as, for instance, one of the bile 

 salts — instead of an inorganic compound is dissolved in the fluid, the surface 

 tension of the air-water surface is reduced, and in consequence the bile salt is 

 concentrated at that surface; wlTile in the remainder of the fluid, and particu- 

 larly in that portion of it in contact with the wall of the vessel, the concentra- 

 tion is reduced. 



The distribution of a salt in such a fluid, whether it lowers surface tension or 

 increases it, is due to the action of a law which may be expressed in words to 

 the effect that the concentration in a system is so adjusted as to reduce the energy 

 at any point to a minimum. 



Our knowledge of this action of inorganic and organic substances on the 

 surface tension in a fluid and of the differences in their concentrations throughout 

 the latter was contained in the results of the observations on gas mixtures by 

 J. Willard Gibbs, published in 1878. The principle as applied to solutions was 

 independently discovered by J. J. Thomson in 1887. It is known as the Gibbs' 

 principle, although the current enunciations of it contain the more extended 

 observations of Thomson. As formulated usually it is more briefly given, and 

 its essential points may be rendered in the statement that, when a substance, on 

 solution in a fluid lowers the surface tension of the latter the concentration of the 

 solute is greater in the surface layer than elsewhere, in the solution; bvt when the 

 substance dissolved raises the surface tension of the fluid, the concentration of 

 the solute is least in the surface layers of the solution. 



It is thus seen how in a system like that of a drop of water with different 

 contact surfaces the surface tension is affected, and how this alters the distribu- 

 tion of solutes. It is further to be noted that for most organic solutes the action 

 in this respect is the very reverse of that of inorganic salts. Consequently, in a 

 living cell which contains both inorganic and organic solutes, and in which there 

 are portions of different composition and density, the equilibrium may be subject 

 to disturbance constantly through an alteration of the surface tension at any point. 

 Such a disturbance may be found in a drop of an emulsion of olive oil and 

 potassium carbonate in the well-known experiments of Butschli. When the 

 emulsion is appropriately prepared, a minute drop of it, after it is surrounded with 

 water, will creep under the cover glass in an amoeboid fashion for hours, and 

 the movement will be more marked and rapid when the temperature is raised to 

 40° to 50° C. All the phenomena manifested are due to a lowering of the surface 

 tension at a point on the surface, as a result of which there is protrusion there 

 of the contents of the drop, accompanied, Butschli holds, by streaming cyclic 

 currents in the remainder of the mass. 



Surface tension also, according to J. Traube, is all-important in osmosis, and 

 he holds that it is the solution pressure (Haftdruclc) of a substance which 

 determines the velocity of the osmotic movement and the direction and force 

 of the osmotic pressure. The solution pressure of a substance is measured by 

 the effect that substance exercises when dissolved on the surface tension of its 

 solution, or, to put it in Traube's own way, the more a substance lowers or raises 

 the surface tension of a solvent (water) the less or greater is the solution pressure 

 {Haftdruck) of that substance. This solution pressure, Traube further holds, is 

 the only force controlling osmosis through a membrane, and he rejects completely 

 the bombardment effect on the septum postulated in the van't Hoff theory of 

 osmosis. 



The question as to the nature of the factors concerned in osmosis must remain 

 undecided until the facts have been more fully studied from the physiological 

 standpoint, but enough is now known to indicate that surface tension plays at 

 least a part in it, and the omission of all consideration of it as a factor is not by 

 any means a negligible defect in the van't Hoff theory of osmosis. 



The occurrence of variations in surface tension in the individual cells of an 

 organ or tissue is difficult to demonstrate directly. We have no methods for that 

 purpose, and, in consequence, one must depend on indirect ways to reveal whether 

 such variations exist. The most effective of these is to determine the distribution 

 of organic solutes and of inorganic salts in the cell. The demonstration of the 

 former is at present difficult, or even in some cases impossible. The occurrence 

 of soaps which are amongst the most effective agents in lowering surface tension 

 may be revealed without difficulty microchemically, as may also neutral fats ■ 



