Drops of Oil floating on Water. 289 
from the theory of the molecular forces of fluids contained in 
my communication to the February Number of this Journal. 
It is there supposed that the sphere of action of the attractive 
molecular forces of fluids is much greater than that of the re- 
pulsive, and that the latter increase so rapidly with any de- 
crement of the mutual distances of the molecules, as to be 
taken account of without sensible error by considering the 
fluid incompressible. On this supposition the angle of actual 
contact between a solid and a fluid, or that between two fluids, 
is determined by the hydrostatical equilibrium resulting from 
the molecular attractions of the two substances, the solid like 
the fluid being treated as incompressible. It thence appeared 
that this is an exceedingly small angle in cases in which the 
bodies in contact are not of very different specific gravities. 
Hence in the instance before us, the angle of contact, that is, 
the angle which the surface of contact of the oil and water 
makes with the upper free surface of the oil, is very small. 
Bat since the drop is convex both at its upper and under sur- 
faces, this is apparently an angle of considerable magnitude. 
In fact the theoretical angle of contact, or that which the 
upper surface of the oil makes with an imaginary surface 
drawn parallel to its under surface and just beyond the sphere 
of the molecular action of the water, would be found by cal- 
culation to be of sensible magnitude. Consequently, that the 
angle of actual contact may be exceedingly small, the portion 
of the upper surface of the oil that lies within the sphere of 
the molecular action of the water must undergo a flexure 
near the visible periphery of the drop. Now in fulfilling this 
condition it seems probable that a very thin film of the oil 
spreads over the whole water surface, (as there is no force to 
counteract,) and gives rise at the same time to the visible 
spreading of the first drop. The film itself being of less 
thickness than the radius of the sphere of the molecular action 
of the water, will not be perceptible to the senses. Such a cir- 
cumstance having happened to the drop that first comes in 
contact with the water, will prevent any that succeed from 
being similarly affected. 
I take this opportunity of adverting to the editorial note 
(signed E. W. B.) in the February Number, (p. 172,) on my 
communication in that Number, and thanking the author of 
it for correcting the erroneous assertion that mercury is inca- 
pable of adhering to solids, which was inconsiderately made 
of solids in general, when I was more particularly referring 
to glass. In accordance with the authorities quoted in the 
note, the theory I was explaining would lead to the inference 
Third Series. Vol. 8. No. 47. April 1836. 2H 
