ALTERATIONS IN SURFACE TENSIONS 49 



to this is a layer of ncjjjativcly charged hydroxyls. If these two 

 opposing charges balance one another, the shape of the globule will 

 be defined by surface forces alone, i.e., the surface will be reduced 

 to a minimum and the globule will appear spherical, except, of 

 course, at the mercury-glass interface. Now increase the con- 

 centration of positive ions in the water by adding a drop of sulphuric 

 acid to it, and so reducing the number of effective electrons on the 

 surface of the water adjacent to the mercury, and the balance 

 between protons and electrons is disturbed in favour of the former. 



ABC 



Fig. 10. — Form assumed by mercury globules under different electrical states. _B is a 

 mercury globule immersed in dilute HjS'Oi. It assumes a more spherical form A when con- 

 nected with the negative pole of a battery, while connection to the positive pole reduces 

 surface tension as shown at C. 



The surface forces are, therefore, partially overcome l)y the 

 electrostatic repulsion of similarly charged surface molecules and 

 the globule will flatten out (Fig. 10, B). Increasing the positive 

 charge on the mercury by connecting it to the positive pole of a 

 battery and immersing the lead from the negative element in the 

 acidulated water will cause the surface forces to be still further 

 reduced (Fig. 10, C). The reverse effect occurs when the direction 

 of the electrical current is reversed. A very neat demonstration 

 of this, due to Ostwald, may be given by actually making the 

 mercury one of the elements in a galvanic couple. (Part II., p. 517.) 



CORK. 



5EWIN6 NEtCLE 



.-WATCH GU\iS 



GLOBULE OF MERCURV 



Fig. 11. — Mercury " Heart." 



In this experiment (Fig. 11) the mercury forms the positive 

 element and the carbon in the steel needle the negative element. 

 When connection is made between the + and — substances by 

 placing the needle so that it just touches the mercury the charge on 

 the mercury surface is decreased, the globule more nearly assumes 

 the spherical form and so breaks connection with the needle. This 

 allows the positive charge on the metal to accumulate, lower the 

 surface tension, and so again make contact. (To pre\'ent the 

 cessation of this rhythmic movement by polarisation, a little 

 potassium bichromate is added to the fluid.) 



