Electrification at Liquid-Gas Surfaces. 311 



have a value in a freshly formed surface different from its 

 value in an older surface. One may refer to the two types 

 of experiment as the dynamical and statical, in the same 

 way that one refers to corresponding methods of measuring 

 surface- tensions of solutions* Not only is there a gradual 

 adsorption of salt into the surface on the liquid side, but there 

 may be an adsorption of gas on the opposite side. In a 

 dynamical experiment the surface will not be in that equi- 

 librium state which exists in a statical one. 



Again, it is possible that the gas does not play the same 

 part in producing the electriti cation in the two cases. For 

 example, it is not improbable that a sphere of water-vapour 

 in water would be found to be charged in the statical experi- 

 ment, while a drop of water falling through its own vapour 

 is known to give no electrical effect. In the static case H 

 and show very slight differences in charge, but in dynamic 

 experiments they give rise to charges of opposite sign. The 

 fact that so many substances in contact with water are 

 negatively charged indicates that water by itself produces 

 an electrification upon which is superposed an electrification 

 due to the substance in contact with it. It is probable that 

 in the static experiment the hydrogen and oxygen exert a 

 very small influence on the charge observed, but in dynamic 

 experiments they play an important part. When these 

 gases are torn away from the water by the mechanical 

 forces, the violence of the separation may give rise to an 

 electrification which is not apparent in the statical observa- 

 tions and which depends as to its sign and amount upon the 

 nature of the gas. 



The changes in the electrification produced by dissolved 

 salts are net easy to explain, the difficulty being increased 

 by a lack of knowledge of the nature of the ions formed and 

 the amount of hydrolysis which occurs. Where there is 

 complete dissociation-^as in the dilute solutions here used — - 

 the ions formed would tend to charge the surface with their 

 own sign. This tendency, however, is stronger in some ions 

 than in others, and the same ion does not exhibit the same 

 power to charge different surfaces. The H-ion, for example, 

 shows quite different powers of precipitation toward colloidal 

 As 2 S 3 , gold-sol, mastic and egg-white (Mines, Koll-Chem. 

 Beilu iii. p. 193, 1911-12). In the case of liquid-gas 

 surfaces the H-ion does not seem to show a very strong- 

 charging power, as indicated by the acids HC1 and HN0 8 . 



Of the three trivalent nitrates of La, Al, Or, the first shows 

 scarcely any hydrolysis (Perrin, loc* cit,), and one might 

 expect the effects observed to be due to the simple ions La 



