﻿460 M. G. Quincke on the Capillary Phenomena 



drops of water can be placed on a surface of mercury, and that 

 they, as Draper* and Paalzowf have shown, contract and assume 

 smaller diameters if an electric current is caused to enter the 

 drop of water and pass out through the mercury. The lenticular 

 drops, on the contrary, spread themselves out when the electric 

 current flows in the opposite direction. Even very weak electric 

 currents I have found produce this effect. The contraction of 

 the drops increases with the duration and strength of the elec- 

 tric current. It is seen from this that the nature of the bound- 

 ing surface has a real influence on the shape of the drop of water, 

 that traces of a third substance brought to the separating sur- 

 face of mercury and water (as the small quantities of hydrogen 

 and oxygen liberated here by electrolysis) influence the form of 

 the drop, even if from their small quantity they can scarcely be 

 perceived by other methods of observation. 



Paalzow has shown that contraction of the drop of water 

 can be produced by the addition of hyposulphite of soda to 

 the water, and by the addition of chromic acid an expansion 

 of the drop. He assumes that even the purest mercury contains 

 oxygen and a small quantity of oxide on its surface ; by means 

 of hydrogen electrolytically separated, or the addition of hyposul- 

 phite of soda, this layer of oxide is reduced, the capillary relation 

 of the mercury to water is altered, the repulsion between the two 

 liquids is increased, and thus the contraction of the drop is 

 brought about. 



If this explanation were correct, it would follow that the mer- 

 cury examined in § 8 must have been coated with a layer of 

 oxide, and the numbers in Table X. § 10 for mercury and water 

 must refer to mercury coated with a layer of oxide, for mercury 

 and hyposulphite of soda to mercury with a pure surface ; and 

 it would still be unexplained why drops of water and of hypo- 

 sulphite of soda do not spread on drops of mercury. 



But the contradiction immediately disappears if pure mercury 

 be applied, not containing a trace of a foreign substance, parti- 

 cularly of a fatty or etherial oil. This is obtained in the follow- 

 ing manner. 



The mercury is digested for a long time under frequent agita- 

 tion with concentrated sulphuric acid, to which a couple of drops 

 of nitric acid are added, in order to free it from an admixture of 

 oxide and foreign metals. It is afterwards separated from the 

 sulphuric acid and the insoluble salts by being run repeatedly 

 through a funnel made of clean writing-paper whose aperture is 

 not too small, and which is stuck together on the outside with 



* Phil. Mag. vol. xxvi. p. 187 (1845). In this memoir, however, the 

 direction of the current is reversed. 

 t Pogg. Ann. vol. civ. p. 420 (1858). 



