OF VITAL PHENOMENA 65 



Billard and Bruyant (1905) observed the use made of surface 

 tension by the beetles, Stenus tarsalis and cicendeloides, that dart 

 about on the surface of water. They are pulled forward by the 

 surface tension of the water in front, while the surface tension 

 of the water behind them is decreased by substances discharged 

 from the anus. A model of this may be made by attaching a 

 piece of camphor to one end of a stick floated on water. Traces 

 of the camphor dissolve and lower the surface tension behind, 

 and the stick moves forward. An irregular piece of camphor 

 alone darts about owing to unequal solution on different sides. 



Adsorption 

 As a rule, substances which lower the tension of the water- 

 air surface also lower the surface tension between water and 

 a solid, but the latter can be measured only in terms of adsorp- 

 tion. Traube gives the following data showing the effect of .25 

 molecular solutions on the tension of the water-air surface as 

 compared with the amount adsorbed by a unit quantity of animal 

 charcoal. 



Solute. .Surface tension. Adsorption. 



Acetic acid .84 .244 



Ethyl alcohol .85 .136 



Propionic acid -7S7S .366 



Butyric acid .613 .573 



i-Amyl alcohol .374 .786 



It may be seen that surface tension and adsorption are inversely 

 proportional except in the case of acetic acid, which is adsorbed 

 by charcoal more than we would expect from the surface tension 

 of its solution against air. A similar series was obtained of the 

 adsorption of these substances by powdered glass, cotton and silk. 

 The adsorption curve for different concentrations is similar to 

 the surface tension curve, Fig. 24, a larger proportion of the 

 solute being adsorbed from dilute solutions than from more con- 

 centrated solutions. An empirical formula to express this curve 



x 

 that has been suggested is — = ac 1 '/", where x is the amount of 



m 



substance adsorbed, m is the amount of adsorbent powder, c is 

 the concentration of the solution after equilibrium is established 

 and a and n are empirical constants (Freundlich, 1907). N is 

 usually greater than 1 and this makes the equation characteristic 



