﻿of the common Surface of two Liquids. 463 



shaped drop whose diameter quickly diminishes, since the ten- 

 sion of the surface of the mercury is more quickly changed than 

 that of the water, in consequence of this condensation. The angle 

 between water and mercury changes with the quantity and nature 

 of the thin layer of liquid which has formed itself out of the con- 

 densed moisture and has coated the mercury. This also ex- 

 plains the different form of the lens-shaped drop of water which 

 is generally observed on the mercury, unless certain precaution- 

 ary measures have been taken. 



A flat lens-shaped water-drop frequently contracts on the (im- 

 pure) surface of mercury when the latter is breathed on. If air 

 be passed through Babo's pearl-tubes wetted with water, this 

 effect is considerably weakened; hence it proceeds from the 

 foreign substances in the moisture of the air breathed. After a 

 short time they disappear, and the drop of water again assumes 

 its original form. 



I believe that the layer of a liquid, the thickness of which does 

 not exceed one-millionth of a millimetre, can be perceived in this 

 manner, and that this method under favourable circumstances 

 even surpasses in sensitiveness the one depending on optical 

 phenomena. 



Drops of an aqueous solution of hyposulphite of soda always 

 assumed the shape of a lens on the cold surface of mercury in 

 the experiments which I made. It must remain doubtful whether 

 the non-appearance of the spreading is to be ascribed to the im- 

 purity of the mercury or to small errors in the numbers of 

 Table X. § 10. Owing to the great difficulty of these researches, 

 I cannot assert that perfectly pure mercury was used; and the 

 question can only be decided by an expenditure of time and 

 means which, I regret to say, I cannot at present command. The 

 impurities of the mercury,, in case such were actually present, 

 cannot have had a material influence on the remaining experi- 

 ments of this communication or the conclusions derived from 

 them. 



If the common boundary of mercury and water be covered 

 with electrolytically separated gaseous hydrogen, the capillary 

 tension of this limiting surface will approach the sum of the capil- 

 lary tensions of a free surface of mercury and a free surface of 

 water (that is_, the magnitudes 55*03 + 8*25 = 63*28 milligrms.), 

 and will be so much greater than the capillary tension 42*58 of 

 the common contact-surface of water and mercury the greater 

 the quantity of hydrogen liberated on this surface. Hence the 

 contraction of the drop of water must increase with the quantity 

 of hydrogen liberated, as experiment also shows. 



The layer of oxide formed by means of the liberated oxygen 

 on the mercury surface diminishes the capillary tension of the 



