﻿of the common Surface of two Liquids, 459 



the capillary constants* are 131*5 and 79*75 milligrms. respec- 

 tively, a trace of leadf be placed, whose capillary constant is 

 45*66 milligrms. at 330°, and at 1200° or 1000° is consequently 

 considerably less, the height of the drop and K— k are im- 

 mediately seen to decrease considerably, as theory requires. 

 The melted lead (with smaller capillary constant) at once coats 

 the whole drop, and the tension of the surface becomes consider- 

 ably less than before. 



The magnitude K — k, which for pure gold and silver amounts 

 to 4 millims., decreases here 1 millim. or more. There is pos- 

 sibly superficial oxidation of the lead coating, although the surface 

 of the drops retains its metallic appearance. 



Flat drops of copper covered with a very thin layer of oxide 

 are much shallower than those with a pure surface, because the 

 oxide of copper which covers the surface has a much smaller ca- 

 pillary constant than copper. In this case a 12 is assumed =0 

 for the limit of copper and oxide of copper, since, from Matthies- 

 sen's experiments J, the oxide appears to dissolve in the metal. 



The layer which induces this change in the form of the drop 

 is so remarkably thin that it is completely beyond all other me- 

 thods of observation, spectral analysis perhaps excepted§. 



If a substance when placed on a liquid is very quickly dissolved 

 by the latter, it is easy to understand that the change in shape 

 of the capillary surface may not occur. This is the case, for ex- 

 ample, when oil of turpentine is placed on flat drops of bisulphide 

 of carbon in water (compare § 14). 



Small quantities of tin, copper, or silver placed on flat drops 

 of melted gold, or tin on flat drops of melted silver, do not ma- 

 terially change the form of these drops. 



Thus these metals appear to be dissolved more quickly than 

 lead by melted gold and silver. I may perhaps have even 

 overlooked a trifling change of form, as I was obliged to make 

 this experiment on comparatively small drops from scarcity of 

 pure substance. 



27. From the magnitude of the capillary constants of the com- 

 mon surface of mercury and the liquids adduced in the first sec- 

 tion (Table X. § 10), it follows that a ]2 is always less than a, — a 2 , 

 that consequently, according to equation (3) § 25, all the liquids 

 named in that Table, especially water and an aqueous solution 

 of hyposulphite of soda, must spread themselves out on a free 

 surface of mercury. 



In apparent contradiction to this is the fact that lenticular 



* Pogg. Ann. vol. cxxxviii. p. 148 (1869). 



t Ibid. vol. cxxxv. p. 642 (186.9). 



X Ibid. vol. ex. p. 224 (1860). 



§ Compare Pogg. Ann. vol. cxxxviii. p. 147 (1869). 



