b} 
Fuly 29, 1886] 
and glass beakers, and shown on an enlarged scale in 
these two diagrams (Figs. 6 to 8); which represent 
bisulphide of carbon floating on the surface of sulphate of 
zinc, and in this case (Fig. 8) the bisulphide of carbon 
drop is of nearly the maximum size capable of floating. 
Here is the bottle whose contents are represented in Fig. 
8, and we shall find that a very slight vertical disturb- 
ance serves to submerge the mass of bisulphide of 
carbon. There now it has sunk, and we shall find when 
its vibrations have ceased that the bisulphide of carbon 
has taken the form of a large sphere supported within 
the sulphate of zinc. Now, remembering that we are 
again at the centre of the earth, and that gravity does 
not hinder us, suppose the glass matter of the bottle 
suddenly to become liquid sulphate of zinc, this mass 
would become a compound sphere like the one shown 
on this diagram (Fig. 3), and would have a radius of 
about 8 centimetres. If it were sulphate of zine alone, 
and of this magnitude, its period of vibration would be 
about 54 seconds. 
Fig. 9 shows a drop of sulphate of zinc floating on a 
wine-glassful of bisulphide of carbon. 
In observing the phenomena of two liquids in contact, 
I have found it very convenient to use sulphate of zinc 
(which I find, by experiment, has the same free-surface 
tension as water) and bisulphide of carbon; as these 
NATURE 
293 
liquids do not mix when brought together, and, for 
a short time at least, there is no chemical interaction 
between them. Also, sulphate of zinc may be made 
to have a density less than, or equal to, or greater 
than, that of the bisulphide, and the bisulphide may be 
coloured to a more or less deep purple tint by iodine, and 
this enables us easily to observe drops of any one of these 
liquids on the other. In the three bottles now before 
you the clear liquid is sulphate of zinc—in one bottle it 
has a density less than, in another equal to, and in the 
third greater than, the density of the sulphide—and you 
see how, by means of the coloured sulphide, all the phe- 
nomena of drops resting upon or floating within a liquid 
into which they do not diffuse may be observed, and, 
under suitable arrangements, quantitatively estimated. 
When a liquid under the influence of gravity is sup- 
ported by a solid, it takes a configuration in which the 
difference of curvature of the free surface at different 
levels is equal to the difference of levels divided by the 
surface tension reckoned in terms of weight of unit bulk 
of the liquid as unity ; and the free surface of the liquid 
leaves the free surface of the solid at the angle whose 
Fic. 1x 
' cosine is, as stated above, equal to the interfacial 
tension divided by the free-surface tension, or at an angle 
of 180° in any case in which minus the interfacial tension 
exceeds the free-surface tension. The surface equation 
of equilibrium and the boundary conditions thus stated 
in words, suffice fully to determine the configuration when 
the volume of the liquid and the shape and dimensions of 
the solid are given. When I say determine, I do not 
mean unambiguously. There may of course be a multi- 
plicity of solutions of the problem ; as, for instance, when 
the solid presents several hollows in which, or projections 
hanging from which, portions of the liquid, or in or 
hanging from any one of which the whole liquid, may 
rest. 
When the solid is symmetrical round a vertical axis, 
the figure assumed by the liquid is that of a figure of 
revolution, and its form is determined by the equation 
given above in words. A general solution of this problem 
by the methods of the differential and integral calculus 
transcends the powers of mathematical analysis, but the 
following simple graphical method of working out what 
constitutes mathematically a complete solution, occurred 
to me a great many years ago. 
Draw a line to represent the axis of the surface of revo- 
