79 
on the Cohesion of Fluids, 
by half the sum of the elevation and depression on the remote 
sides of the substances, and as the distance increases, this 
maximum is only diminished by a quantity, which is initially 
as the square of the distance. The figures of the solids con- 
cerned modify also sometimes the law of attraction, so that, 
for bodies surrounded by a depression, there is sometimes a 
maximum, beyond which the force again diminishes : and it is 
hence that a light body floating on mercury, in a vessel little 
larger than itself, is held in a stable equilibrium without 
touching the sides. The reason of this will become apparent, 
when we examine the direction of the surface necessarily 
assumed by the mercury in order to preserve the appropriate 
angle of contact, the tension acting with less force when the 
surface attaches itself to the angular termination of the float 
in a direction less horizontal. 
The apparent attraction produced between solids by the 
interposition of a fluid does not depend on their being partially 
immersed in it; on the contrary, its effects are still more power- 
fully exhibited in other situations ; and, when the cohesion 
between two solids is increased and extended by the interven- 
tion of a drop of water or of oil, the superficial cohesion of 
these fluids is fully sufficient to explain the additional effect. 
When wholly immersed in water, the cohesion between two 
pieces of glass is little or not at all greater than when dry : 
but if a small portion only of a fluid be interposed, the curved 
surface, that it exposes to the air, will evidently be capable of 
resisting as great a force as it would support from the pressure 
of the column of fluid that it is capable of sustaining in a ver- 
tical situation ; and in order to apply this force, we must employ 
in the separation of the plates, as great a force as is equivalent 
