October 7, 1910] 



SCIENCE 



455 



eate that the distribution of the potassium 

 is closely associated with contraction, and, 

 therefore, with the production of energy in 

 contractile tissues. The condensation of 

 potassium at a point may, of course, be a 

 result of a combination with portions of 

 the cytoplasm, but we have no knowledge 

 of the occurrence of such compounds ; and, 

 further, the presence of such does not ex- 

 plain anything, or account for the libera- 

 tion of energy in motor contraction. On 

 the other hand, the action of surface ten- 

 sion would explain not only the localiza- 

 tion of the potassium but also the libera- 

 tion of the energy. 



In vessels holding fluids the latter, in re- 

 lation to surface tension, have two surfaces 

 — one free, in contact with the air, and 

 known as the air- water surface; the other, 

 that in contact with the wall of the con- 

 taining vessel (glass). In the latter the 

 tension is lower than in the former. "When 

 an inorganic compoimd — a salt, for ex- 

 ample — is dissolved in the fluid it increases 

 the tension at the air-water surface, but its 

 dilution is much greater here than in any 

 other part of the fluid; while at the other 

 surface its concentration is greatest. In 

 the latter case the condition is of the nature 

 of adsorption. The condensation on that 

 portion of the surface where the tension is 

 least is responsible for what we find when 

 a solution of a colored salt, as, e. g., potas- 

 sixvoa permanganate, is driven through a 

 layer of dry sand. If the latter is of some 

 considerable thickness the fluid as it passes 

 out is colorless. The air-solution surface 

 tension is higher than the tension of each 

 of the solution-sand surfaces on which, 

 therefore, the permanganate condenses or 

 is adsorbed. The same phenomenon is ob- 

 served when a long strip of filter paper is 

 allowed to hang with its lower end in con- 

 tact with a moderately dilute solution of a 

 copper salt. The solution is imbibed by 



the filter paper, and it ascends a certain 

 distance in a couple of minutes, when it 

 may be found that the uppermost portion 

 of the moist area is free from even a trace 

 of copper salt. 



If, on the other hand, an organic com- 

 pound — as, for instance, one of the bile 

 salts — instead of an organic compound is 

 dissolved in the fluid, the surface tension 

 of the air-water surface is reduced, and in 

 conseciuence the bile salt is concentrated at 

 that surface ; while in the remainder of the 

 fliiid, and particularly in that portion of 

 it in contact with the wall of the vessel, the 

 concentration is reduced. 



The distribution of a salt in such a fluid, 

 whether it lowers surface tension or in- 

 creases 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 siibstances on the surface ten- 

 sion in a fluid and of the differences in 

 their concentrations throughout the latter 

 was contained in the results of the observa- 

 tions on gas mixtures by J. Willard Gibbs, 

 published in 1878. The principle as ap- 

 plied to solutions was independently dis- 

 covered by J. J. Thomson in 1887. It is 

 known as the Gibbs 's 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 ren- 

 dered in the statement that when a sub- 

 stance on solution in a fluid lowers the sur- 

 face tension of the latter the concentration 

 of the solute is greater in the surface layer 

 than elsewhere in the solution; but 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. 



