de placed upon clean water is
also, at first sight, an exception to Marangoni's rule. So far from
spreading over the surface, as according to its lower surface-tension it
ought to do, it remains suspended in the form of a lens. Any dust that
may be lying upon the surface is not driven away to the edge of the
drop, as would happen in the case of oil. A simple modification of the
experiment suffices, however, to clear up the difficulty. If after the
deposition of the drop, a little lycopodium be scattered over the
surface, it is seen that a circular space surrounding the drop, of about
the size of a shilling, remains bare, and this, however often the
dusting be repeated, so long as any of the carbon bisulphide remains.
The interpretation can hardly be doubtful. The carbon bisulphide is
really spreading all the while, but on account of its volatility is
unable to reach any considerable distance. Immediately surrounding the
drop there is a film moving outwards at a high speed, and this carries
away almost instantaneously any dust that may fall upon it. The
phenomenon above described requires that the water-surface be clean. If
a very little grease be present, there is no outward flow and dust
remains undisturbed in the immediate neighbourhood of the drop.]

[Illustration: FIG. 6.]

_On the Rise of a Liquid in a Tube_.--Let a tube (fig. 6) whose internal
radius is r, made of a solid substance c, be dipped into a liquid a. Let
us suppose that the angle of contact for this liquid with the solid c is
an acute angle. This implies that the tension of the free surface of the
solid c is greater than that of the surface of contact of the solid with
the liquid a. Now consider the tension of the free surface of the liquid
a. All round its edge there is a tension T acting at an angle a with the
vertical. The circumference of the edge is 2[pi]r, so that the resultant
of this tension is a force 2[pi]rT cos[alpha] acting vertically upwards
on the liquid. Hence the liquid will rise in the tube till the weight
of the vertical column between the free surface and the level of the
liquid in the vessel balances the resultant of the surface-tension. The
upper surface of this column is not level, so that the height of the
column cannot be directly measured, but let us assume that h is the mean
height of the column, that is to say, the height of a column of equal
weight, but with a flat top. Then if r is the radius of the tube at the
top of the c