CAPILIARY ACTION. £53 



ready seen, tliut the density of all bodies Is a little diminish- Capillary ac- 

 ca\ in the neighbourhood of an cxpo?.cd surface ; and when ^^^" ° "* ' 



these bodies come into, contact with others, new varieties 

 may again arise, from tlie efi^^ct of the mutual cohesion ou 

 the superficial density. 



We may also obtain, from this important law, a very dis'^ 

 tinct idea of the manner in which a drop of a lighter liquid 

 spreads itself on a heavier :. the sum of the tensions of the 

 two surfaces of the drop being abrays smaller than the ten* 

 siort Bf the stirrounding surface.' When, however, one drop 

 has thu^ diffused itself, it leaves an invisible film on the 

 surface, which reduces the tension tOL an. equality with that 

 of the drop : and in this state of the surface, a new drop 

 will remain undisturbed. Thfe form of the lower surface of 

 the drop will be of the same nature with that of the upper, 

 its curvature being proportional to the height above a cer- 

 tain line, at which the internal and external pressure would 

 become equal. It cannot therefore happen, except by acci- 

 dent, that the surface of the fluid should remain perfectly 

 horizontal, although it may require a very nice examination 

 to detect its curvature. For example, in the case of oil 

 floating fon water, the tension of the surface of the oil is 

 only about half as great as that of the water, consequently 

 the tension of the common surface must be about one fourth, 

 or half as great as that of the oil : but the inclination of this 

 surface to the horizon, when the drop is thin, is more than 

 ten times as great as that of the upppr surface; consequently 

 the direction of the joint efl'ect of the tension must be in- 

 clined to the horizon in an angle about one fourth as great 

 as that which is contained by the surfaces of the drop; and 

 the contiguous surface of the water must assume a similar 

 direction in order to counteract this force : this will require 

 a shght depression of the drop, and consequently a slight 

 alteration of its curvature, but the general result will be the 

 same. On the supposition of a force simply proportional to 

 the difference of densitj'^ or of capillary force, a still greater 

 derangement of the general surface would, in this case, be 

 required : so that the phenomena of drops, floating in 'this 

 manner, have not, in fact, any tendency to establish such a 

 law, although they may appear at first sight to favour it. 



The 



