48 CAPILLARY RIPPLES. 
and exactly adjusted in the manner which we all look upon as being 
necessary in a lecture. Under those circumstances you will, I am sure, 
excuse any very evident shortcomings either in adjustment or in the 
time needed for getting the things sufficiently visible for it to be 
possible for me to show what I mean. 
Coming now to my subject, I wish first of all to shortly dwell upon 
the fact that the surfaces of all liquids behave as though they were 
elastic ; that is to say in the case of water, say, the surface of the water 
is as though it were a very thin sheet of something like stretched india 
rubber, and it always tends to shrink up and become smaller. The 
same is true in the case of mercury, melted metals and oils, and of any 
liquid whatever. ‘This force with which the surface of a liquid tends 
to contract, which is ordinarily called surface tension, gives rise to the 
whole series of capillary phenomena, one of which alone to-day, or at 
the outside two, [ wish to enlarge upon. ‘They are called capillary, not 
that they have to do directly with hairs or with the curling or other- 
wise of hair, which has been suggested, but because the phenomena 
which this elastic surface renders most evident are those which are 
produced when very fine tubes, as fine as a hair, are made use of. 
And_ now I should like to employ some hairs of an ordinary camel’s- 
hair brush to illustrate this very point. I will show you an old experi- 
ment; one which if it were not called an experiment would be recognised 
as a familiar experience. The brush is projected upon the screen ; you 
see the hairs are distinct, separate and sharply defined. It is dry. 
It is ordinarily stated that if you dip a brush into water, wet it and 
shake out the superfluous water, the hairs of the brush are drawn 
together at the point, because the brush is wet. It is because the 
brush is wet in a sense, but that is not a sufficient explanation, because 
if the brush is again put into water it is still wet and being in the water 
the hairs separate exactly as they did before. It is when the brush is 
out of the water so that the water on the brush is the outside surface 
of the water in which this capillary or elastic contractile force exists, 
it is only then that the hairs are drawn together. (Illustrating the 
same by electric light on a screen). Iam only showing that one experi- 
ment out of a great number—because time will not admit of more—to 
illustrate the fact that the surface of a liquid always tends with some 
considerable force to become smaller than it actually is. To give the 
figures, if you draw a line on the surface of clean water an inch long 
then the surface on either side of that line is pulling away from the line 
with a force equal to the weight of about three grains or a little more, 
not a very large force. In the case of melted metals it is more; in the 
case of mercury it is eight or ten times as great; in the case of 
aluminium it is very great, and so drops of melted aluminium are much 
larger than corresponding drops of mercury or water. Other materials, 
such for example, as oils, benzine, bi-sulphide of carbon, and a great 
number of organic liquids have a much smaller surface tension. I wish 
now, before I come to deal with one of the influences of this force, to 
show experimentally, by an exceedingly transient experiment I am 
afraid, that in the case of aluminium, at any rate, this force is capable 
of holding together a melted wire for an appreciable time, long enough, 
