GIFT OF
MICHAEL REESE
,-
SOAP-BUBBLES
AND THE
FORCES WHICH MOULD THEM,
)RNife
THE ROMANCE OF SCIENCE.
SOAP-BUBBLES
AND THE
FORCES WHICH MOULD THEM
BEING A COURSE OF THREE LECTURES
DELIVERED IN THE THEATRE OF THE LONDON
INSTITUTION ON THE AFTERNOONS OF DEC. 30, 1889,
JAN. I AND 3, 1890, BEFORE A JUVENILE AUDIENCE.
BY
C. V. BOYS, A.R.S.M., F.R.S.,
ASSISTANT PROFESSOR OF PHYSICS AT THE ROYAL COLLEGE OF SCIENCE,
SOUTH KENSINGTON.
PUBLISHED UNDER THE DIRECTION OF THE GENERAL LITERATURE
COMMITTEE.
XTNIVERSITT
SOCIETY FOR PROMOTING CHRISTIAN KNOWLEDGE,
LONDON : NORTHUMBERLAND AVENUE, W.C. J
43, QUEEN VICTORIA STREET, E.G.
BRIGHTON: 129, NORTH STREET.
NEW YORK: E. &J. B. YOUNG & CO-
1896.
TO
G. F. RO DWELL,
THE FIRST
SCIENCE-MASTER APPOINTED AT MARLBOROUGH COLLEGE,
BY THE AUTHOR
AS A TOKEN OF ESTEEM AND GRATITUDE,
AND IN THE HOPE THAT
IT MAY EXCITE IN A FEW YOUNG PEOPLE SOME SMALL
FRACTION OF THE INTEREST AND ENTHUSIASM WHICH
HIS ADVENT AND HIS LECTURES AWAKENED
IN THE AUTHOR, UPON WHOM THE LIGHT
OF SCIENCE THEN SHONE FOR
THE FIRST TIME.
or THE
UNIVERSITY*;
PREFACE.
I WOULD ask those readers who have grown
up, and who may be disposed to find fault with
this book, on the ground that in so many
points it is incomplete, or that much is so
elementary or well known, to remember that
the lectures were meant for juveniles, and
for juveniles only. These latter I would urge
to do their best to repeat the experiments
described. They will find that in many cases
no apparatus beyond a few pieces of glass or
india-rubber pipe, or other simple things easily
obtained are required. If they will take this
trouble they will find themselves well repaid,
and if instead of being discouraged by a few
failures they will persevere with the best means
at their disposal, they will soon find more to
interest them in experiments in which they
only succeed after a little trouble than in those
Vlll PREFACE.
which go all right at once. Some are so
simple that no help can be wanted, while some
will probably be too difficult, even with assist-
ance ; but to encourage those who wish to see
for themselves the experiments that I have
described, I have given such hints at the end
of the book as I thought would be most
useful.
I have freely made use of the published
work of many distinguished men, among
whom I may mention Savart, Plateau, Clerk
Maxwell, Sir William Thomson, Lord Ray-
leigh, Mr. Chichester Bell, and Prof. Rucker.
The experiments have mostly been described
by them, some have been taken from journals,
and I have devised or arranged a few. I am
also indebted to Prof. Rucker for the use of
various pieces of apparatus which had been
prepared for his lectures.
SOAP-BUBBLES,
AND THE
FORCES WHICH MOULD THEM.
I DO not suppose that there is any one in
this room who has not occasionally blown a
common soap-bubble, and while admiring the
perfection of its form, and the marvellous
brilliancy of its colours, wondered how it is
that such a magnificent object can be so easily
produced.
I hope that none of you are yet tired of
playing with bubbles, because, as I hope we
shall see during the week, there is more in a
common bubble than those who have only
played with them generally imagine.
The wonder and admiration so beautifully
portrayed by Millais in a picture, copies of
IO SOAP-BUBBLES, AND
which, thanks to modern advertising enterprise,
some of you may possibly have seen, will, I
hope, in no way fall away in consequence of
these lectures; I think you will find that it
will grow as your knowledge of the subject
increases. You may be interested to hear that
we are not the only juveniles who have played
with bubbles. Ages ago children did the
same, and though no mention of this is made
by any of the classical authors, we know that
they did, because there is an Etruscan vase in
the Louvre in Paris of the greatest antiquity,
on which children are represented blowing
bubbles with a pipe. There is however, no
means of telling now whose soap they used.
It is possible that some of you may like
to know why I have chosen soap-bubbles
as my subject ; if so, I am glad to tell you.
Though there are many subjects which might
seem to a beginner to be more wonderful,
more brilliant, or more exciting, there are few
which so directly bear upon the things which
we see every day. You cannot pour water
from a jug or tea from a tea-pot; you can-
not even do anything with a liquid of any
kind, without setting in action the forces to
THE FORCES WHICH MOULD THEM. II
which I am about to direct your attention.
You cannot then fail to be frequently re-
minded of what you will hear and see in
this room, and, what is perhaps most im-
portant of all, many of the things I am
going to show you are so simple that you
will be able without any apparatus to repeat
for yourselves the experiments which I have
prepared, and this you will find more inter-
esting and instructive than merely listening to
me and watching what I do.
There is one more thing I should like to
explain, and that is why I am going to show
experiments at all. You will at once answer
because it would be so dreadfully dull if I
didn't. Perhaps it -x would. But that is not
the only reason. <J^\^gjE^mind you then
that when we want to find out anything that
we do not know, there are two ways of pro-
ceeding. _JWe_m ay either ask somebody else
who does know, or read^whaTthe most learned
men have written about it, which is a very good
plan if anybody happens to be able to answer
our question ; or else we may adopt the other
plan, and by arranging an experiment, try
for ourselves. An experiment is a question
12 SOAP-BUBBLES, AND
which we ask of Nature, who is always ready
to give a correct answer, provided we ask
properly, that is, provided we arrange a proper
experiment. An experiment is not a conjuring
trick, something simply to make you wonder,
nor is it simply shown because it is beautiful,
or because it serves to relieve the monotony
of a lecture ; if any of the experiments I show
are beautiful, or do serve to make these lec-
tures a little less dull, so much the better;
but their chief object is to enable you to see
for yourselves what the true answers are to
questions that I shall ask.
Now I shall begin by performing an experi-
ment which you have all probably tried dozens
of times. I have in my hand a common
camers-hair brush. If you want to make the
hairs cling together and come to a point, you
wet it, and then you say the hairs cling to-
gether because the brush is wet. Now let us
try the experiment ; but as you cannot see
this brush across the room, I hold it in front
of the lantern, and you can see it enlarged
upon the screen (Fig. i, left hand). Now it
is dry, and the hairs are separately visible. I
am now dipping it in the water, as you can
THE FORCES WHICH MOULD THEM. 13
see, and on taking it out, the hairs, as we
expected, cling together (Fig. i, right hand),
because they are wet, as we are in the habit
of saying. I shall now hold the brush in
the water, but there it is evident that the
Fig. i.
hairs do not cling at all (Fig. i, middle),
and yet they surely are wet now, being actually
in the water. It would appear then that the
reason which we always give is not exactly
correct. This experiment, which requires no-
thing more than a brush and a glass of
water, then shows that the hairs of a brush
cling together not only because they are
wet, but for some other reason as well
which we do not yet know. It also shows
that a very common belief as to opening our
eyes under water is not founded on fact. It
is very commonly said that if you dive into
the water with your eyes shut you cannot see
properly when you open them under water,
because the water gums the eyelashes down
over the eyes ; and therefore you must dive in
with your eyes open if you wish to see under
water. Now as a matter of fact this is not
the case at all ; it makes no difference whether
your eyes are open or not when you dive in,
you can open them and see just as well either
way. In the case of the brush we have seen
that water does not cause the hairs to cling
together or to anything else when under the
water, it is only when taken out that this is
the case. This experiment, though it has not
explained why the hairs cling together, has at
any rate told us that the reason always given
is not sufficient.
I shall now try another experiment as simple
as the last. I have a pipe from which water
*
THE FORCES WHICH MOULD THEM. 15
is very slowly issuing, but it does not fall
away continuously ; a drop forms which slowly
grows until it has attained a certain definite
size, and then it suddenly falls away. I want
you to notice that every time this happens
the drop is always exactly the same size and
shape. Now this cannot be mere chance;
there must be some reason for the definite size,
and shape. Why does the water remain at
all? It is heavy and is ready to fall, but it
does not fall; it remains clinging until it is
a certain size, and then it suddenly breaks
away, as if whatever held it was not strong
enough to carry a greater weight. Mr. Worth-
ington has carefully drawn on a magnified
scale the exact shape of a drop of water of
different sizes, and these you now see upon
the diagram on the wall (Fig. 2). These
diagrams will probably suggest the idea that
the water is hanging suspended in an elastic
bag, and that the bag breaks or is torn away
when there is too great a weight for it to
carry. It is true there is no bag at all really,
but yet the drops take a shape which suggests
an elastic bag. To show you that this is no
fancy, I have supported by a tripod a large
1 6 SOAP-BUBBLES, AND
ring of wood over which a thin sheet of india-
rubber has been stretched, and now on allowing
water to pour in from this pipe you will see the
rubber slowly stretching under the increasing
weight, and, what I especially want you to
Fig. 2.
notice, it always assumes a form like those on
the diagram. As the weight of water increases
the bag stretches, and now that there is about
a pailful of water in it, it is getting to a
state which indicates that it cannot last much
longer; it is like the water-drop just before
THE B'oRCES WHICH MOULD THEM. 17
it falls away, and now suddenly it changes its
shape (Fig. 3), and it would immediately tear
Fig- 3-
itself away if it were not for the fact that india-
rubber does not stretch indefinitely ; after a
time it gets tight and will withstand a greater
l8 SOAP-BUBBLES, AND
pull without giving way. You therefore see
the great drop now permanently hanging which
is almost exactly the same in shape as the
water-drop at the point of rupture. I shall
now let the water run out by means of a
syphon, and then the drop slowly contracts
again. Now in this case we clearly have a
heavy liquid in an elastic bag, whereas in the
drop of water we have the same liquid but no
bag that is visible. As the two drops behave
in almost exactly the same way, we should
naturally be led to expect that their form and
movements are due to the same cause, and that
the small water-drop has something holding it
together like the india-rubber you now see.
Let us see how this fits the first experiment
with the brush. That showed that the hairs
do not cling together simply because they are
wet ; it is necessary also that the brush should
be taken out of the water, or in other words
it. is necessary that the surface or the skin of
the water should be present to bind the hairs
together. If then we suppose that the surface
of water is like an elastic skin, then both the
experiments with the wet brush and with the
water-drop will be explained.
THE FORCES WHICH MOULD THEM. 19
Let us therefore try another experiment to
see whether in other ways water behaves as if
it had an elastic skin.
I have here a plain wire frame fixed to a
stem with a weight at the bottom, and a hollow
glass globe fastened to it with sealing-wax.
The globe is large enough to make the whole
thing float in water with the frame up in the
air. I can of course press it down so that the
frame touches the water. To make the move-
ment of the frame more evident there is fixed
to it a paper flag.
Now if water behaves as if the surface were
an elastic skin, then it should resist the upward
passage of the frame which I am now holding
below the surface. I let go, and instead of
bobbing up as it would do if there were no such
action, it remains tethered down by this skin of
the water. If I disturb the water so as to let
the frame out at one corner, then, as you see, it
dances up immediately (Fig. 4). You can see
that the skin of the water must have been fairly
strong, because a weight of about one quarter
of an ounce placed upon the frame is only just
sufficient to make the whole thing sink.
This apparatus which was originally described
SOAP-BUBBLES, AND
by Van der Mensbrugghe I shall make use of
again in a few minutes.
I can show you in a more striking way that
there is this elastic layer or skin on pure clean
water. I have a small sieve made of wire
gauze sufficiently coarse to
allow a common pin to be
put through any of the
holes. There are moreover
about eleven thousand of
these holes in the bottom
of the sieve. Now, as you
know, clea,n wire is wetted
by water, that is, if it is
dipped in water it comes
out wet ; on the other hand,
some materials, such as
paraffin wax, of which
paraffin candles are made,
are not wetted or really
touched by water, as you
may see for yourselves if you will only dip a
paraffin candle into water. I have melted a
quantity of paraffin in a dish and dipped this
gauze into the melted paraffin so as to coat
the wire all over with it, but I have shaken
Fig. 4.
THE FORCES WHICH MOULD THEM. 21
it well while hot to knock the paraffin out of
the holes. You can now see on the screen that
the holes, all except one or two, are open, and
that a common pin can be passed through
readily enough. This then is the apparatus.
Now if waterTTas an elastic skin which it re-
quires force to stretch, it ought not to run
through these holes very readily; it ought
not to be able to
get through at all
unless forcedy be-
cause at eachTiole
the skin would
liave^jtp.-'ISe stretch-
ed to allow the
water to ger~to the
Fig. 5.
other "side. r This
you understand is
only true if the water does not wet or really
touch the wiret Now to prevent the water
that I am going to pour ' in-frnm striking the
bottom with so much force as to drive it
through, I have laid a small piece of paper
in the sieve, and am pouring the water on to
the paper, which breaks the fall (Fig. 5). I
have now poured in about half a tumbler of
22 SOAP-BUBBLES, AND
water, and I might put in more. I take away
the paper but not a drop runs through. If
I give the sieve a jolt then the water is driven
to the other side, and in a moment it has all
escaped. Perhaps this will remind you of
one of the exploits of our old friend Simple
Simon,
11 Who went for water in a sieve,
But soon it all ran through."
But you see if you only manage the sieve
properly, this is not quite so absurd as people
generally suppose.
If now I shake the water off the sieve, I can,
for the same reason, set it to float on water,
because its weight is not sufficient to stretch
the skin of^the water through all the holes.
The water, therefore, remains on the other side,
and it floats even though, as I have already
said, there are eleven thousand holes in the
bottom, any one of which is large enough to
allow an ordinary pin to pass through. This
experiment also illustrates how difficult it is to
write real and perfect nonsense.
You may remember one of the stories in
Lear's book of Nonsense Songs.
THE FORCES WHICH MOULD THEM.
23
They went to sea in a sieve, they did,
In a sieve they went to sea :
In spite of all their friends could say,
On a winter's morn, on a stormy day,
In a sieve they went to sea.
* * * *
" They sailed away in a sieve, they did,
In a sieve they sailed so fast,
With only a beautiful pea-green veil,
Tied with a riband by way of a sail,
To a small tobacco-pipe mast ;
# # * #
And so on. You see that it is quite pos-
sible to go to sea in
a sieve that is, if
the sieve is large
enough and the water
is not too rough and
that the above lines
are now realized in
every particular (Fig.
6). '
I may give one more
example of the power
of this elastic skin of
water. If you wish
to pour water from a
tumbler into a narrow- Fig. 6.
-
24 SOAP-BUBBLES, AND
necked bottle, you know how if you pour
slowly it nearly all runs down the side of the
glass and gets spilled about, whereas if you
pour quickly there is no room for the great
quantity of water to pass into the bottle all at
once, and so it gets spilled again. But if you
take a piece of stick or a glass rod, and hold it
against the edge of the
tumbler, then the water
runs down the rod and
into the bottle, and none
is lost (Fig. 7) ; you may
even hold the rod inclined
to one side, as I am now
doing, but the water runs
down the wet rod because
this elastic skin forms a
kind of tube which pre-
vents the water from escap-
ing. This action is often made use of in the
country to carry the water from the gutters
under the roof into a water-butt below. A
piece of stick does nearly as well as an iron
pipe, and it does not cost anything like so
much.
I think then I have now done enough to
THE FORCES WHICH MOULD THEM. 25
show that on the surface of water there is a
kind of elastic skin. I do not mean that there
is anything that is not water on the surface,
but that the water while there acts in a different
way to what it does inside,' and that it acts as if
it were an elastic skin made of something like
very thin India- rubber, only that it is perfectly
and absolutely elastic, which india-rubber is not.
You witt now" be in a position to understand
how it is that in narrow tubes water does not
find its own levelfbut behaves in an unexpected
manner.- I have placed in front of the lantern
a dish of water coloured blue so that you may
the more easily see it." I shall now dip into
the water a very narrow glass pipe, and immedi-
ately the water rushes upland stands about half
an inch above the general levdy The tube
inside is wet^ The elastic skinjsf the water is
therefore attached to the tube,* and goes on pull-
ing up the water until the weight of the water
raised"above the general level is equal to the
force- exerted by the skin. If I take a tube
about twice as big, then this pulling action
which is going on all round the tube will cause
it to lift twice the weight of water, but this will
not make the water rise twice as high, because
26 SOAP-BUBBLES, AND
the larger tube holds so much more water for a
given length than the smaller tube. It will not
even pull it up as high as it did in the case of
the smaller tube, because if it were pulled up
as high the weight of the water raised would
in that case be four times as great, and not
only twice as great, as you might at first think.
It will therefore only raise the water in the larger
tube to half the height, and now that the two
tubes are side by side you see the water in the
smaller tube standing twice as high as it does
in the larger tube. In the same way, if I were
to take a tube as fine as a hair the water would
go up ever so much higher. It is for this
reason that this is called Capillarity, from the
Latin word capillus, a hair, because the action
is so marked in a tube the size of a hair.
Supposing now you had a great number of
tubes of all sizes, and placed them in a row
with the smallest on one side and all the others
in the order of their sizes, then it is evident
that the water would rise highest in the smallest
tube and less and less high in each tube in the
row (Fig. 8), until when you came to a very
large tube you would not be able to see that
the water was raised at all. You can very
THE FORCES WHICH MOULD THEM. 2j
easily obtain the same kind of effect by simply
taking two square pieces of window glass and
placing them face to face with a common
match or small fragment of anything to keep
them a small distance apart along one edge
Fig. 8.
while they meet together along the opposite
edge. An india-rubber ring stretched over
them will hold them in this position. I now
take this- pair of plates and stand it in a dish of
coloured water, and you at once see that the
water creeps up to the top of the plates on
28 SOAP BUBBLES, AND
the edge where they meet, and as the distance
between the plates gradually increases, so the
height to which the water rises gradually gets
less, and the result is that the surface of the
liquid forms a beautifully regular curve which
Fig. 9.
is called by mathematicians a rectangular
hyperbola (Fig. 9). I shall have presently
to say more about this and some other curves,
and so I shall not do more now than state
that the hyperbola is formed because as the
width between the plates gets greater the
THE FORCES WHICH MOULD THEM. 19
height gets less, or, what comes to the same
thing, because the weight of liquid pulled up
at any small part of the curve is always the
same.
If the plates or the tubes had been made of
material not wetted by water, then the effect
of the tension of the surface would be to drag
the liquid away from the narrow spaces, and
the more so as the spaces were narrower. As
it is not easy to show this well with paraffined
glass plates or tubes and water, I shall use
another liquid which does not wet or touch
clean glass, namely, quicksilver. As it is not
possible to see through quicksilver, it will not
do to put a narrow tube into this liquid to
show that the level is lower in the tube than
in the surrounding vessel, but the same result
may be obtained by having a wide and a
narrow tube joined together. Then, as you
see upon the screen, the quicksilver is lower in
the narrow than in the wide tube, whereas in
a similar apparatus the reverse is the case with
water (Fig. 10).
I want you now to consider what is happen-
ing when two flat plates partly immersed in
water are held close together. We have seen
3O SOAP-BUBBLES, AND
that the water rises between them. Those
parts of these two plates, which have air
between them and also air outside them (in-
dicated by the letter a in Fig. n), are each of
them pressed equally in opposite directions by
Fig. 10.
the pressure of the air, and so these parts do
not tend to approach or to recede from one
another. These parts again which have water
on each side of each of them (as indicated by
the letter c) are equally pressed in opposite
directions by the pressure of the water, and so
THE FORCES WHICH MOULD THEM. 3!
these parts do not tend to approach or to
recede from one another. But those parts of
the plates (b) which have water between them
and air outside would, you might think, be
pushed apart by the water between them with
a greater force than that which could be
exerted by the air outside, and so you might
m
Fig. II.
be led to expect that on this account a pair of
plates if free to move would separate at once.
But such an idea though very natural is wrong,
and for this reason. The water that is raised
between the plates being above the general
level must be under a less pressure, because,
as every one knows, as you go down in water
3^ SOAP-BUBBLES, AND
the pressure increases, and so as you go up
the pressure must get less. The water then
that is raised between the plates is under a
less pressure than the air outside, and so on
the whole the plates are pushed together.
You can easily see that this is the case. I
have two very light hollow glass beads such
as are used to decorate a Christmas tree.
These will float in water if one end is stopped
with sealing-wax. These are both wetted by
water, and so the water between them is
slightly raised, for they act in the same way as
the two plates, but not so powerfully. How-
ever, you will have no difficulty in seeing that
the moment I leave them alone they rush
together with considerable force. Now if you
refer to the second figure in the diagram,
which represents two plates which are neither
of them wetted, I think you will see, without
any explanation from me, that they should be
pressed together, and this is made evident by
experiment. Two other beads which have been
dipped in paraffin wax so that they are neither
of them wetted by water float up to one another
again when separated as though they attracted
each other just as the clean glass beads did.
THE FORCES WHICH MOULD THEM. 33
If you again consider these two cases, you
will see that a plate that is wetted tends to
move towards the higher level of the liquid,
whereas one that is not wetted tends to move
towards the lower level, that is if the level of
the liquid on the two sides is made different
by capillary action. Now suppose one plate
wetted and the other not wetted, then, as the
diagram imperfectly shows, the level of the
liquid between the plates where it meets the
non-wetted plate is higher than that outside,
while where it meets the wetted plate it is
lower than that outside ; so each plate tends
to go away from the other, as you can see now
that I have one paraffined and one clean ball
floating in the same water. They appear to
repel one another.
You may also notice that the surface of the
liquid near a wetted plate is curved, with the
hollow of the curve upwards, while near a ncai-
wetted plate the reverse is the case. That this
curvature of the surface is of the first import-
ance I can show you by a very simple experi-
ment, which you can repeat at home as easily
as the last that I have shown. I have a clean
glass bead floating in water in a clean glass
34
vessel, which is not quite full. The bead
always goes to the side of the vessel. It is
impossible to make it remain in the middle, it
always gets to one side or the other directly.
I shall now gradually add water until the level
of the water is rather higher than that of the
edge of the vessel. The surface is then
rounded near the vessel, while it is hollow near
the bead, and now the bead sails away towards
the centre, and can by no possibility be made
to stop near either side. With a paraffined
bead the reverse is the case, as you would
expect. Instead of a paraffined bead you may
use a common needle, which you will find
will float on water in a tumbler, if placed upon
it very gently. If the tumbler is not quite
full the needle will always go away from the
edge, but if rather over-filled it will work up
to one side, and then possibly roll over the
edge ; any bubbles, on the other hand, which
were adhering to the glass before will, the
instant that the water is above the edge of the
glass, shoot away from the edge in the most
sudden and surprising manner. This sudden
change can be most easily seen by nearly
filling the glass with water, and then gradually
THE FORCES WHICH MOULD THEM. 35
dipping in and taking out a cork, which will
cause the level to slowly change.
So far I have given you no idea what force
is exerted by this elastic skin of water. Measure-
ments made with narrow tubes, with drops,
and in other ways, all show that it is almost
exactly equal to the weight of three and a
quarter grains to the inch. We have, more-
over, not yet seen whether other liquids act in
the same way, and if so whether in other cases
the strength of the elastic skin is the same.
You now see a second tube identical with
that from which drops of water were formed,
but in this case the liquid is alcohol. Now
that drops are forming, you see at once that
while alcohol makes drops which have a definite
size and shape when they fall away, the alcohol
drops are not by any means so large as the
drops of water which are falling by their side.
Two possible reasons might be given to ex-
plain this. Either alcohol is a heavier liquid
than water, which would account for the smaller
drop if the skin in each liquid had the same
strength, or else if alcohol is not heavier than
water its skin must be weaker than the skin of
water. As a matter of fact alcohol is a lighter
36 SOAt>-BtJBBLES, AND
liquid than water, and so still more must the
skin of alcohol be weaker than that of water.
We can easily put this to the test of experi-
ment. In the game that is called the tug-of-
war you know well enough which side is the
strongest ; it is the side which pulls the other
over the line. Let us then make alcohol and
0-285 Inch
water play the same game. In order that you
may see the water, it is coloured blue. It is
lying as a shallow layer on the bottom of this
white dish. At the present time the skin of
the water is pulling equally in all directions,
and so nothing happens ; but if I pour a few
drops of alcohol into the middle, then at the
line which separates the alcohol from the water
XTNIVEI
THE FORCES WHICJt MOULD fHEM. 37
we have alcohol on one side pulling in, while
we have water on the other side pulling out,
and you see the result. The water is victori-
ous ; it rushes away in all directions, carrying a
quantity of the alcohol away with it, and leaves
the bottom of the dish dry (Fig. 13).
Fig. 13-
This difference in the strength of the skin
of alcohol and of water, or of water containing
much or little alcohol, gives rise to a curious
motion which you may see on the side of a
wine-glass in which there is some fairly strong
wine, such as port. The liquid is observed to
38 SOAP-BUBBLES, AND
climb up the sides of the glass, then to gather
into drops, and to run down again, and this
goes on for a long time. This is explained as
follows : The thin layer of wine on the side
of the glass being exposed to the air, loses
its alcohol by evaporation more quickly than
the wine in the glass. It therefore be-
comes weaker in alcohol or stronger in water
than that below, and for this reason it has a
stronger skin. It therefore pulls up more
wine from below, and this goes on until there
is so much that drops form, and it runs back
again into the glass, as you now see upon the
screen (Fig. 14). There can be no doubt
that this movement is referred to in Proverbs
xxiii. 31: "Look not thou upon the wine
when it is red, when it giveth his colour in
the cup, when it moveth itself aright."
If you remember that this movement only
occurs with strong wine, and that it must have
been known to every one at the time that these
words were written, and used as a test of the
strength of wine, because in those days every
one drank wine, then you will agree that this
explanation of the meaning of that verse is the
right one. I would ask you also to consider
THE FORCES WHICH MOULD THEM. 39
whether it is not probable that other passages
which do not now seem to convey to us any
meaning whatever, may not in the same way
have referred to the common knowledge and
Fig. 14.
customs of the day, of which at the present
time we happen to be ignorant.
Ether, in the same way, has a skin which
is weaker than the skin of water. The very
smallest quantity of ether on the surface of
water will produce a perceptible effect. For
instance, the wire frame which I left some
4O SOAP-BUBBLES, AND
time ago is still resting against the water-skin.
The buoyancy of the glass bulb is trying to
push it through, but the upward force is just
not sufficient. I will however pour a few
drops of ether into a glass, and simply pour
the vapour upon the surface of the water (not
a drop of liquid is passing over), and almost
immediately sufficient ether has condensed
upon the water to reduce the strength of the
skin to such an extent that the .frame jumps
up out of the water.
There is a well-known case in which the
difference between the strength of the skins
of two liquids may be either a source of
vexation or, if we know how to make use of
it, an advantage. If you spill grease on your
coat you can take it out very well with benzine.
Now if you apply benzine to the grease, and
then apply fresh benzine to that already there,
you have this result there is then greasy
benzine on the coat to which you apply fresh
benzine. It so happens that greasy benzine
has a stronger skin than pure benzine. The
greasy benzine therefore plays at tug-of-war
with pure benzine, and being stronger wins and
runs away in all directions, and the more you
THE FORCES WHICH MOULD THEM. 41
apply benzine the more the greasy benzine
runs away carrying the grease with it. But
if you follow the directions on the bottle, and
first make a ring of clean benzine round the
grease-spot, and then apply benzine to the
grease, you then have the greasy benzine run-
ning away from the pure benzine ring and
heaping itself together in the middle, and
escaping into the fresh rag that you apply, so
that the grease is all of it removed.
There is a difference again between hot and
cold grease, as you may see, when you get
home, if you watch a common candle burning.
Close to the flame the grease is hotter than it
is near the outside. It has therefore a weaker
skin, and so a perpetual circulation is kept up,
and the grease runs out on the surface and
back again below, carrying little specks of
dust which make this movement visible, and
making the candle burn regularly.
You probably know how to take out grease-
stains with a hot poker and blotting-paper.
Here again the same kind of action is going
on.
A piece of lighted camphor floating in water
is another example of movement set up by
42 SOAP-BUBBLES, AND
differences in the strength of the skin of water
owing to the action of the camphor.
I will give only one more example.
If you are painting in water-colours on
greasy paper or certain shiny surfaces the paint
will not lie smoothly on the paper, but runs
together in the well-known way ; a very little
ox-gall, however, makes it lie perfectly, because
ox-gall so reduces the strength of the skin of
water that it will wet surfaces that pure water
will not wet. This reduction of the surface
tension you can see if I use the same wire
frame a third time. The ether has now
evaporated, and I can again make it rest against
the surface of the water, but very soon after I
touch the water with a brush containing ox-gall
the frame jumps up as suddenly as before.
It is quite unnecessary that I should any
further insist upon the fact that the outside of
a liquid acts as if it were a perfectly elastic
skin stretched with a certain definite force.
Suppose now that you take a small quantity
of water, say as much as would go into a nut-
shell, and suddenly let it go, what will happen ?
Of course it will fall down and be dashed
against the ground. Or again, suppose you
THE FORCES WHICH MOULD THEM. 43
take the same quantity of water and lay it
carefully upon a cake of paraffin wax dusted
over with lycopodium which it does not wet,
what will happen ? Here again the weight of
the drop that which makes it fall if not held
will squeeze it against the paraffin and make
it spread out into a flat cake. What would
happen if the weight of the drop or the force
pulling it downwards could be prevented from
acting ?, In such a case the drop would only
feel the effect of the elastic skin, which would
try to pull it into such a form as to make
the surface as small as possible. It would
in fact rapidly become a perfectly round ball,
because in no other way can so small a sur-
face be obtained. If, instead of taking so much
water, we were to take a drop about as large
as a pin's head, then the weight which tends
to squeeze it out or make it fall would be far
less, while the skin would be just as strong,
and would in reality have a greater moulding
power, though why I cannot now explain.
We should therefore expect that by taking a
sufficiently small quantity of water the mould-
ing power of the skin would ultimately be able
almost entirely to counteract the weig-ht of the
44 SOAP-BUBBLES, AND
drop, so that very small drops should appear like
perfect little balls. If you have found any diffi-
culty in following this argument, a very simple
illustration will make it clear. You many of
you probably know how by folding paper to
make this little thing which I hold in my
hand (Fig. 15). It is called a cat-box, because
of its power of dispelling cats when it is filled
Fig.
with water and well thrown. This one, large
enough to hold about half a pint, is made out
of a small piece of the Times newspaper.
You may fill it with water and carry it about
and throw it with your full power, and the
strength of the paper skin is sufficient to hold
it together until it hits anything, when of
course it bursts and the water comes out. On
THE FORCES WHICH MOULD THEM. 45
the other hand, the large one made out of a
whole sheet of the Times is barely able to
withstand the weight of the water that it will
hold. It is only just strong enough to allow
of its being filled and carried, and then it
may be dropped from a height, but you can-
not throw it. In the same way the weaker
skin of a liquid will not make a large quantity
take the shape of a ball, but it will mould a
minute drop so perfectly that you cannot tell
by looking at it that it is not perfectly round
every way. This is most easily seen with
quicksilver. A large quantity rolls about like
a flat cake, but the very small drops obtained
by throwing some violently on the table and
so breaking it up appear perfectly round.
You can see the same difference in the beads
of gold now upon the screen (Fig. 16). They
are now solid, but they were melted and
then allowed to cool without being disturbed.
Though the large bead is flattened by its
weight, the small one appears perfectly round.
Finally, you may see the same .thing with
water if you dust a little lycopodium on the
table. Then water falling will roll itself up
into perfect little balls. You may even see
46 SOAP-BUBBLES, AND
the same thing on a dusty day if you water
the road with a water-pot.
If it were not for the weight of liquids, that
is the force with which they are pulled down
towards the earth, large drops would be as
Fig. 16.
perfectly round as small ones. This was first
beautifully shown by Plateau, the blind experi-
mentalist, who placed one liquid inside another
which is equally heavy, and with which it does
not mix. Alcohol is lighter than oil, while
water is heavier, but a suitable mixture of alcohol
THE FORCES WHICH MOULD THEM. 47
and water is just as heavy as oil, and so oil does
not either tend to rise or to fall when immersed
in such a mixture. I have in front of the
lantern a glass box containing alcohol and
water, and by means of a tube I shall slowly
allow oil to flow in. You see that as I remove
the tube it becomes a perfect ball as large as a
walnutr There are now two or three of these
balls of oil all perfectly round. I want you to
notice that when I hit them on one side the
large balls recover their shape slowly, while the
small ones become rooHid again much more
quickly. Thsre is a v^ry beautiful effect which
can be produced with this apparatus, and though
it is not necessary to refer to it, it is well
worth while now that the apparatus is set up
to show it to you. In the middle of the box
there is an axle with a disc upon it to which I
can make the oil adhere. Now if I slowly turn
the wire and disc the oil will turn also. As I
gradually increase the speed the oil tends to fly
away in all directions, but the elastic skin
retains it. The result is that the ball becomes
flattened at its poles like the earth itself. On
increasing the speed, the tendency of the oil to
get away is at last too much for the elastic skin,
48 SOAP-BUBBLES, AND
and a ring breaks away (Fig. 17), which almost
immediately contracts again on to the rest of
the ball as the speed falls. If I turn it suffi-
ciently fast the ring breaks up into a series of
balls which you now see. One cannot help
Fig. 17.
being reminded of the heavenly bodies by this
beautiful experiment of Plateau's, for you see a
central body and a series of balls of different
sizes all travelling round in the same direction
(Fig. 1 8) ; but the forces which are acting in
THE FORCES WHICH MOULD THEM. 49
the two cases are totally distinct, and what you
see has nothing whatever to do with the sun
and the planets.
We have thus seen that a large ball of liquid
can be moulded by the elasticity of its skin if
Fig. 18.
the disturbing effect of its weight is neutral-
ized, as in the last experiment. This disturbing
effect is practically of no account in the case
of a soap-bubble, because it is so thin that it
hardly weighs anything. You all know, of
5<D SOAP-BUBBLES.
course, that a soap-bubble is perfectly round,
and now you know why; it is because the
elastic film, trying to become as* small as it
can, must take the form which has the smallest
surface for its content, and that form is the
sphere. I want you to notice here, as with the
oil, that a large ^bubble oscillates" much more
slowly than a small one when knocked out of
shape with a-bat^covered with baize or wool.
The chief result that I have endeavoured to
make clear to-day is this. The outside of a
liquid acts as if it were an elastic skin, which
will, as far as it is able, so mould the liquid
within it that it shall be as small as possi
Generally the weight of liquids, especially wl
there is a large quantity, is too much for the
feebly elastic skin, and its power may not be
noticed. The disturbing effect of weight is got
rid of by immersing one liquid in another
which is equally heavy with which it do v es" not
mix, and it is hardly noticed when very small
drops are examined, or when a bubble is blown,
for in these cases the weight is almost nothing,
while the elastic power of the skin is just as
great as ever.
LECTURE II.
I DID not in the last lecture by any direct
experiment show that a soap-film or bubble is
really elastic, like a pjece of stretched india-
rubber*
A, soap-^bubble^ consisting, as it does, of a thin
layer of liquid, which must have of course both
an inside and an outside surface or skin, must
be elastic, and this is easily shown in many
ways. Perhaps the easiest way is to tie a
thread across a ring rather loosely, and then to
dip the ring into soap water. On taking it
out there is a film stretched over the ring, in
which the thread moves about quite freely, as
you can see upon the screen. But if I break
the film on one side, then immediately the
thread is pulled by the film on the other side
as far as it can go, and it is now tight (Fig.
19). You will also notice that it is part of a
perfect circle, because that form makes the
space on one side as great, and therefore on
the other side, where the film is, as small,
as possible. Or again, in this second ring the
thread is double for a short distance in the
middle. If I break the film between the
Fig. 19.
threads they are at once pulled apart, and are
pulled into a perfect circle (Fig. 20), because
that is the form which makes the space within
it as great as possible, and therefore leaves the
space outside it as small as possible. You will
also notice, that though the circle will not
THE FORCES WHICH MOULD THEM. 53
allow itself to be pulled out of shape, yet it
can move about in the ring quite freely, because
such a movement does not make any difference
to the space outside it.
I have now blown a bubble upon a ring
Fig. 20.
of wire. I shall hang a small ring upon it,
and to show more clearly what is happening,
I shall blow a little smoke into the bubble.
Now that I have broken the film inside the lower
ring, you will see the smoke being driven out
and the ring lifted up, both of which show the
54
SOAP-BUBBLES, AND
elastic nature of the film. Or again, I have
blown a bubble on the end of a wide pipe ; on
holding the open end of the pipe to a candle
flame, the outrushing air blows out the flame
at- once, which shows that the soap-bubble is
Fig. 21.
acting like an elastic bag (Fig. 21). You
now see that, owing to the elastic skin of a
soap-bubble, the air inside is under pressure
and will get out if it can. Which would you
think would squeeze the air inside it most, a
large or a small bubble ? We will find out by
THE FORCES WHICH MOULD THEM. 55
trying, and then see if we can tell why. You
now see two pipes each with a tap. These are
joined together by a third pipe in which there
is a third tap. I will first blow one bubble
and shut it off with the tap I (Fig. 22), and
Fig. 22.
then the other, and shut it off with the tap 2.
They are now nearly equal in size, but the air
cannot yet pass from one to the other because
the tap 3 is turned off. Now if the pressure
in the largest one is greatest it will blow
air into the other when I open this tap 9
56 SOAP-BUBBLES, AND
until they are equal in size; if, on the other
hand, the pressure in the small one is greatest,
it will blow air into the large one, and will itself
get smaller until it has quite disappeared. We
will now try the experiment. You see imme-
diately that I open the tap 3 the small bubble
shuts up and blows out the large one, thus
showing that there is a greater pressure in a
small than in a large bubble. The directions
in which the air and the bubble move is in-
dicated in the figure by arrows. I want you
particularly to notice and remember this,
because this is an experiment on which a
great deal depends. To impress this upon
your memory I shall show the same thing in
another way. There is in front of the lantern
a little tube shaped like a U half filled with
water. One end of the U is joined to a pipe on
which a bubble can be blown (Fig. 23). You
will now be able to see how the pressure
changes as the bubble increases in size, because
the water will be displaced more when the pres-
sure is more, and less when it is less. Now
that there is a very small bubble, the pressure
as measured by the water is about one quarter
of an inch on the scale. The bubble is grow-
THE FORCES WHICH MOULD THEM. 57
ing and the pressure indicated by the water in
the gauge is falling, until, when the bubble is
double its former size, the pressure is only
half what it was ; and this is always true, the
Fig. 23.
smaller the bubble the greater the pressure.
As the film is always stretched with the same
force, whatever size the bubble is, it is clear
that the pressure inside can only depend upon
the curvature of a bubble. In the case of
58
lines, our ordinary language tells us, that the
larger a circle is the less is its curvature; a
piece of a small circle is said to be a quick
or a sharp curve, while a piece of a great
circle is only slightly curved ; and if you
take a piece of a very large circle indeed, then
you cannot tell it from a straight line, and you
say it is not curved at all. With a part of the
surface of a ball it is just the same the larger
the ball the less it is curved ; and if the ball is
very large indeed, say 8000 miles across, you
cannot tell a small piece of it from a true
plane. Level water is part of such a surface,
and you know that still water in a basin appears
perfectly flat, though in a very large lake or the
sea you can see that it is curved. We have ]
seen that in large bubbles the pressure is little I
and the curvature is little, while in small bubbles i
the pressure is great and the curvature is great.
The pressure and the curvature rise and fall
together. We have now learnt the lesson
which the experiment of the two bubbles, one
blown out by the other, teaches us.
A ball or sphere is not the only form which
you can give to a soap-bubble. If you take
a bubble between two rings, you can pull it
THE FORCES WHICH MOULD THEM.
59
out until at last it has the shape of a round
straight tube or cylinder as it is called. We
have spoken of the curvature of a ball or
sphere ; now what is the curvature of a cylinder ?
Looked at sideways, the edge of the wooden
cylinder upon the table appears straight, i. e.
not curved at all ; but looked at from above
SIDE
VIEW
Fig. 24.
it appears round, and is seen to have a definite
curvature (Fig. 24). What then is the curva-
ture of the surface of a cylinder? We have
seen that the pressure in a bubble depends upon
the curvature when they are spheres, and this
is true whatever shape they have. If, then, we
find what sized sphere will produce the same
pressure upon the air inside that a cylinder
does, then we shall know that the curvature of
60 SOAP-BUBBLES, AND
the cylinder is the same as that of the sphere
which balances it. Now at each end of a
short tube I shall blow an ordinary bubble,
but I shall pull the lower bubble by means
of another tube into the cylindrical form, and
finally blow in more or less air until the sides
Fig. 25.
of the cylinder are perfectly straight. That is
now done (Fig. 25), and the pressure in the
two bubbles must be exactly the same, as there
is a free passage of air between the two. On
measuring them you see that the sphere is
exactly double the cylinder in diameter. But
THE FORCES WHICH MOULD THEM. 6 1
this sphere has only half the curvature that a
sphere half its diameter would have. Therefore
the cylinder, which we know has the same
curvature that the large sphere has, because
the two balance, has only half the curvature of
a sphere of its own diameter, and the pressure
in it is only half that in a sphere of its own
diameter.
I must now make one more step in explain-
ing this question of curvature. Now that the
cylinder and sphere are balanced I shall blow
in more air, making the sphere larger; what
will happen to the cylinder ? The cylinder is,
as you see, very short ; will it become blown
.out too, or what will happen r Now that I am
blowing in air you see the sphere enlarging,
thus relieving the pressure ; the cylinder
develops a waist, it is no longer a cylinder,
the sides are curved inwards. As I go on blow-
ing and enlarging the sphere, they go on falling
inwards, but not indefinitely. If I were to blow
the upper bubble till it was of an enormous
size the pressure would become extremely
small. Let us make the pressure nothing at
all at once by simply breaking the upper
bubble, thus allowing the air a free passage
62 SOAP-BUBBLES, AND
from the inside to the outside of what was the
cylinder. Let me repeat this experiment on
a larger scale. I have two large glass rings,
between which I can draw out a film of the
Fig. 26.
same kind. Not only is the outline of the
soap-film curved inwards, but it is exactly the
same as the smaller one in shape (Fig. 26).
As there is now no pressure there ought to be
THE FORCES WHICH MOULD THEM. 63
no curvature, if what I have said is correct.
But look at the soap-film. Who would
venture to say that that was not curved ? and
yet we had satisfied ourselves that the pres-
sure and the curvature rose and fell together.
We now seem to have come to an absurd
conclusion. Because the pressure is reduced
to nothing we say the surface must have no
curvature, and yet a glance is sufficient to
show that the film is so far curved as to have
a most elegant waist. Now look at the plaster
model on the table, which is a model of a
mathematical figure which also has a waist.
Let us therefore examine this cast more in
detail. I have a disc of card which has exactly
the same diameter as the waist of the cast. I
now hold this edgeways against the waist
(Fig. 27), and though you can see that it does
not fit the whole curve, it fits the part close to
the waist perfectly. This then shows that this
part of the cast would appear curved inwards
if you looked at it sideways, to the same extent
that it would appear curved outwards if you
could see it from above. So considering the
waist only, it is curved both towards the inside
and also away from the inside according to the
64 SOAP-BUBBLES, AND
way you look at it, and to the same extent.
The curvature inwards would make the pres-
sure inside less, and the curvature outwards
would make it more, and as they are equal
they just balance, and there is no pressure at
all. If we could in the same way examine the
Fig. 27.
bubble with the waist, we should find that this
was true not only at the waist but at every part
of it. Any curved surface like this which at
every point is equally curved opposite ways,
is called a surface of no curvature, and so what
seemed an absurdity is now explained. Now
this surface, which is the only one of the kind
THE FORCES WHICH MOULD THEM. 65
symmetrical about an axis, except a flat sur-
face, is called a catenoid, because it is like a
chain, as you will see directly, and, as you
know, catena is the Latin for a chain. I shall
now hang a chain in a loop from a level stick,
and throw a strong light upon it, so that you
can see it well (Fig. 28). This is exactly the
Fig. 28.
same shape as the side of a bubble drawn
out between two rings, and open at the end
to the air. 1
Let us now take two rings, and having placed
a bubble between them, gradually alter the
pressure. You can tell what the pressure is
1 If the reader finds these geometrical relations too
difficult to follow, he or she should skip the next pages,
and go on again at " We have found . . ." p. 7 7.
66 SOAP-BUBBLES, AND
by looking at the part of the film which
covers either ring, which I shall call the cap.
This must be part of a sphere, and we know
that the curvature of this and the pressure
inside rise and fall together. I have now
adjusted the bubble so that it is a nearly
perfect sphere. If
I blow in more air
the caps become
more curved, show-
ing an increased
pressure, and the
Fig 29 sides bulge out even
more than those of
a sphere (Fig. 29). I have now brought the
whole bubble back to the spherical form. A
little increased pressure, as shown by the
increased curvature of the cap, makes the
sides bulge more; a little less pressure, as
shown by the flattening of the caps, makes
the sides bulge less. Now the sides are
straight, and the cap, as we have already
seen, forms part of a sphere of twice the
diameter of the cylinder. I am still further
reducing the pressure until the caps are plane,
that is, not curved at all. There is now no
THE FORCES WHICH MOULD THEM. 67
pressure inside, and therefore the sides have,
as we have already seen, taken the form of a
hanging chain ; and now, finally, the pressure
inside is less than that outside, as you can
see by the caps being drawn inwards, and the
sides have even a smaller waist than the cate-
noid. We have now seen seven curves as we
gradually reduced the pressure, namely
1. Outside the sphere.
2. The sphere.
3. Between the sphere and the cylinder.
4. The cylinder.
5. Between the cylinder and the catenoid.
6. The catenoid.
7. Inside the catenoid.
Now I am not going to say much more
about all these curves, but I must refer to the
very curious properties that they possess. In
the first place, they must all of them have the
same curvature in every part as the portion of
the sphere which forms the cap ; in the second
place, they must all be the curves of the least
possible surface which can enclose the air and
join the rings as well. And finally, since they
pass insensibly from one to the other as the
pressure gradually changes, though they are
68 SOAP-BUBBLES, AND
distinct curves there must be some curious and
intimate relation between them. Tl though
it is a little difficult, I shall explain. If I were
to say that these curves are the roulettes of
the conic sections I suppose I should alarm
you, and at the same time explain nothing, so
I shall not put it in that way ; but instead, I
shall show you a simple experiment which will
throw some light upon the subject, which you
can try for yourselves at home.
I have here a common bedroom candlestick
with a flat round base. Hold the candlestick
exactly upright near to a white wall, then you
will see the shadow of the base on the wall
below, and the outline of the shadow is a
symmetrical curve, called a hyperbola. Gradu-
ally tilt the candle away from the wall, you
will then notice the sides of the shadow
gradually branch away less and less, and when
you have so far tilted the candle away from
the wall that the flame is exactly above the
edge of the base, and you will know when
this is the case, because then the falling grease
will just fall on the edge of the candlestick and
splash on to the carpet, I have it so now,
the sides of the shadow near the floor will be
THE FORCES WHICH MOULD THEM. 69
almost parallel (Fig. 30), and the shape of the
shadov '11 have become a curve, known as a
parabola; and now when the candlestick is
still more tilted, so that the grease misses the
Fig. 30.
base altogether and falls in a gentle stream
upon the carpet, you will see that the sides of
the shadow have curled round and met on the
wall, and you now have a curve like an oval,
except that the two ends are alike, and this is
70 SOAP-BUBBLES, AND
called an ellipse. If you go on tilting the
candlestick, then when the candle is just
level, and the grease pouring away, the shadow
will be almost a circle ; it would be an exact
circle if the flame did not flare up. Now
if you go on tilting the candle, until at last
the candlestick is upside down, the curves
already obtained will be reproduced in the
reverse order, but above instead of below you.
You may well ask what all this has to do
with a soap-bubble. You will see in a moment.
When you light a candle, the base of the
candlestick throws the space behind it into
darkness, and the form of this dark space,
which is everywhere round like the base, and
gets larger as you get farther from the flame,
is a cone, like the wooden model on the table.
The shadow cast on the wall is of course the
part of the wall which is within this cone. It
is the same shape that you would find if you
were to cut a cone through with a saw, and
so these curves which I have shown you are
called conic sections. You can see some of
them already made in the wooden model on
the table. If you look at the diagram on the
wall (Fig. 31), you will see a complete cone at
THE FORCES WHICH MOULD THEM,
first upright
(A) 3 then being
gradually tilted
over into the
positions that
I have speci-
fied. The black
line in the
upper part of
the diagram
shows where
the cone is cut
through, and
the shaded area
below shows
the true shape
of these shad-
ows, or pieces
cut off, which
are called sec-
tions. Now in
each of these
sections there
are either one
or two points,
each of which
72 SOAP-BUBBLES, AND
is called a focus, and these are indicated by
conspicuous dots. In the case of the circle
(D Fig. 31), this point is also the centre. Now
if this circle is made to roll like a wheel
along the straight line drawn just below it, a
pencil at the centre will rule the straight line
which is dotted in the lower part of the figure ;
but if we were to make wheels of the shapes of
any of the other sections, a pencil at the focus
would certainly not draw a straight line.
What shape it would draw is not at once
evident. First consider any of the elliptic
sections (C, E, or F) which you see on either
side of the circle. If these were wheels, and
were made to roll, the pencil as it moved along
would also move up and down, and the line it
would draw is shown dotted as before in the
lower part of the figure. In the same way the
other curves, if made to roll along a straight
line, would cause pencils at their focal points
to draw the other dotted lines.
We are now almost able to see what the
conic section has to do with a soap-bubble.
When a soap-bubble was blown between two
rings, and the pressure inside was varied, its
outline went through a series of forms, some
THE FORCES WHICH
of which are represented by the dotted lines
in the lower part of the figure, but in every
case they could have been accurately drawn by
a pencil at the focus of a suitable conic section
made to roll on a straight line. I called one
of the bubble forms, if you remember, by its
name, catcnoid ; this is produced when there
is no pressure. The dotted curve in the second
figure B is this one; and to show that this
catenary can be so drawn, I shall roll upon a
straight edge a board made into the form of
the corresponding section, which is called a
parabola, and let the chalk at its focus draw
its curve upon the black board. There is
the curve, and it is as I said, exactly the curve
that a chain makes when hung by its two ends.
Now that a chain is so hung you see that it
exactly lies over the chalk line.
All this is rather difficult to understand,
but as these forms which a soap-bubble takes
afford a beautiful example of the most im-
portant principle of continuity, I thought it
would be a pity to pass it by. It may be put
in this way. A series of bubbles may be blown
between a pair of rings. If the pressures are
different the curves must be different. In
74 SOAP-BUBBLES, AND
blowing them the pressures slowly and continu-
ously change, and so the curves cannot be alto-
gether different in kind. Though they may
be different curves, they also must pass slowly
and continuously one into the other. We find
the bubble curves can be drawn by rolling
wheels made in the shape of the conic sections
on a straight line, and so the conic sections,
though distinct curves, must pass slowly and
continuously one into the other. This we saw
was the case, because as the candle was slowly
tilted the curves did as a fact slowly and in-
sensibly change from one to the other. There
was only one parabola, and that was formed
when the side of the cone was parallel to the
plane of section, that is when the falling grease
just touched the edge of the candlestick;
there is only one bubble with no pressure, the
catenoid, and this is drawn by rolling the para-
bola. As the cone is gradually inclined more,
so the sections become at first long ellipses,
which gradually become more and more round
until a circle is reached, after which they
become more and more narrow until a line is
reached. The corresponding bubble curves
are produced by a gradually increasing pressure,
THE FORCES WHICH MOULD THEM. 75
and, as the diagram shows, these bubble curves
are at first wavy (C), then they become straight
when a cylinder is formed (D), then they be-
come wavy again (E and F), and at last, when
the cutting plane, i. e. the black line in the
upper figure, passes through the vertex of the
cone the waves become a series of semicircles,
indicating the ordinary spherical soap-bubble.
Now if the cone is inclined ever so little more a
new shape of section is seen (G), and this being
rolled, draws a curious curve with a loop in it ;
but how this is so it would take too long to
explain. It would also take .too long to trace
the further positions of the cone, and to trace
the corresponding sections and bubble curves
got by rolling them. Careful inspection of the
diagram may be sufficient to enable you to
work out for yourselves what will happen in all
cases. I should explain that the bubble sur-
faces are obtained by spinning the dotted lines
about the straight line in the lower part of
Fig. 31 as an axis.
As you will soon find out if you try, you
cannot make with a soap-bubble a great length
of any of these curves at one time, but you
may get pieces of any of them with no more
76 SOAP-BUBBLES, AND
apparatus than a few wire rings, a pipe,, and a
little soap and water. You can even see the
whole of one of the loops of the dotted curve
of the first figure (A), which is called a nodoid,
not a complete ring, for that is unstable, but a
part of such a ring. Take a piece of wire or a
match, and fasten one end to a piece of lead, so
that it will stand upright in a dish of soap
water, and project half an inch or so. Hold
with one hand a sheet of glass resting on the
match in middle, and blow a bubble in the
water against the match. As soon as it
touches the glass plate, which should be
wetted with the soap solution, it will become
a cylinder, which will meet the glass plate in
a {rue circle. Now very slowly incline the
plate. The bubble will at once work round
to the lowest side, and try to pull itself away
from the match stick, and in doing so it will
develop a loop of the nodoid, which would be
.exactly true in form if the match or wire were
slightly bent, so as to meet both the glass and
the surface of the soap water at a right angle.
I have described this in detail, because it is
not generally known that a complete loop of
the nodoid can be made with a soap-bubble.
THE FORCES WHICH MOULD THEM. 77
We have found that the pressure in a short
cylinder gets less if it begins to develop a
waist, and greater if it begins to bulge. Let us
therefore try and balance one with a bulge
against another with a waist. Immediately that
I open the tap and let the air pass, the one
Fig. 32.
with a bulge blows air round to the one with
a waist and they both become straight. In
Fig. 32 the direction of the movement of the
air and of the sides of the bubble is indi-
cated by arrows. Let us next try the same
78 SOAP-BUBBLES, AND
experiment with a pair of rather longer cylinders,
say about twice as long as they are wide.
They are now ready, one with a bulge and one
with a waist. Directly I open the tap, and let
the air pass from one to the other, the one with
Fig. 33-
a waist blows out the other still more (Fig. 33),
until at last it has shut itself up. It there-
fore behaves exactly in the opposite way that
the short cylinder did. If you try pairs of
cylinders of different lengths you will find that
the change occurs when they are just over one
THE FORCES WHICH MOULD THEM.
79
and a half times as long as they are wide.
Now if you imagine one of these tubes joined
on to the end of the other, you will see that a
cylinder more than about three times as long
as it is wide cannot last more than a moment;
because if one end were to
contract ever so little the
pressure there would increase,
and the narrow end would
blow air into the wider end _
(Fig. 34), until the sides of
the narrow end met one
another. The exact length
of the longest cylinder that <~
is stable, is a little more than
three diameters. The cylinder
just becomes unstable when
its length is equal to its cir-
cumference, and this is 3*
*ia* ^*t*
diameters almost exactly.
I will gradually separate these rings, keep-
ing up a supply of air, and you will see
that when the tube gets nearly three times
as long as it is wide it is getting very diffi-
cult to manage, and then suddenly it grows
a waist nearer one end than the other, and
8o SOAP-BUBBLES, AND
breaks off forming a pair of separate and
unequal bubbles.
If now you have a cylinder of liquid of great
length suddenly formed and left to itself, it
clearly cannot retain that form. It must break
up into a series of drops. Unfortunately the
changes go on so quickly in a falling stream
of water that no one by merely looking at it
could follow the movements of the separate
drops, but I hope to be able to show to you
in two or three ways exactly what is happen-
ing. You may remember that we were able
to make a large drop of one liquid in another,
because in this way the effect of the weight was
neutralized, and as large drops oscillate or
change their shape much more slowly than
small, it is more easy to see what is happen-
ing. I have in this glass box water coloured
blue on which is floating paraffin, made heavier
by mixing with it a bad-smelling and dangerous
liquid called bisulphide of carbon.
The water is only a very little heavier than
the mixture. If I now dip a pipe into the
water and let it fill, I can then raise it and
allow drops to slowly form. Drops as large
as a shilling are now forming, and when each
THE FORCES WHICH MOULD THEM. 8 1
one has reached its full size, a neck forms
above it, which is drawn out by the falling
drop into a little cylinder.
You will notice that the liquid
of the neck has gathered it-
self into a little drop which
falls away just after the large
drop. The action is now
going on so slowly that you
can follow it. Fig. 35 con-
tains forty-three consecutive
views of the growth and fall of
the drop taken photographic- Sce Dia Z ram at
ally at intervals of one-twen- the end of the
tieth of a second. For the Book.
use to which this figure is to
be put, see page 149. If I
again fill the pipe with water,
and this time draw it rapidly
out of the liquid, I shall leave
behind a cylinder which will
break up into balls, as you
can easily see (Fig. 36). I
should like now to show^you,
as I have this apparatus^in its
place, that you can blow bubbles of water
8 2 SOAP-BUBBLES, AND
containing paraffin in the paraffin mixture,
to
THE FORCES WHICH MOULD THEM. 83
and you will see some which have other
bubbles and drops of one or other liquid inside
again,, One of these compound bubble drops
is now resting stationary on a heavier layer of
liquid, so that you can see it all the better
Fig. 37-
- 37). If I rapidly draw the pipe out of
the box I shall leave a long cylindrical bubble
of water containing paraffin, and this, as was
the case with the water-cylinder, slowly breaks
up into spherical bubbles.
Having now shown that a very large liquid
84 SOAP-BUBBLES, AND
cylinder breaks up regularly into drops, I shall
next go the other extreme, and take as an
example an excessively fine cylinder. You see
Fig. 38.
a photograph of a spider on her geometrical
web (Fig. 38). If I had time I should like
to tell you how the spider goes to work to
make this beautiful structure, and a great deal
HE FORCES WHICH MOULD THEM. 85
about these wonderful creatures, but I must do
no more than show you that there are two
kinds of web those that point outwards, which
are hard and smooth, and those that go round
and round, which are very elastic, and which
are covered with beads of a sticky liquid.
Now there are in a good web over a quarter
of a million of these beads which catch the
flies for the spider's dinner. A spider makes
a whole web in an hour, and generally has to
make a new one every day. She would not
be able to go round and stick all these in
place, even if she knew how, because she would
not have time. Instead of this she makes use
of the way that a liquid cylinder breaks up
into beads as follows. She spins a thread,
and at the same time wets it with a sticky
liquid, which of course is at first a cylinder.
This cannot remain a cylinder, but breaks up
into beads, as the photograph taken with a
microscope from a real web beautifully shows
(Fig. 39). You see the alternate large and
small drops, and sometimes you even see extra
small drops between these again. In order
that you may see exactly how large these
beads really are, I have placed alongside a
86
scale of thousandths of an inch,
which was photographed at the
same time. To prove to you that
this is what happens, I shall now
show you a web that I have made
myself by stroking a quartz fibre
with a straw dipped in castor-oil.
The same alternate large and
small beads are again visible just
as perfect as they were in the
spider's web. In fact it is impos-
sible to distinguish between one
of my beaded webs and a spider's
by looking at them. -And there
is this additional similarity my
webs are just as good as a spider's
for catching flies. You might
say that a large cylinder of water
in oil, or a microscopic cylinder
on a thread, is not the same as
an ordinary jet of water, and that
you would like to see if it be-
5 10
Scale of thousanbths of an Inch
THE FORCES WHICH MOULD THEM. 87
haves as I have described. The next photo-
graph (Fig. 40), taken by the light of an
instantaneous electric spark,
and magnified three and a
quarter times, shows a fine
column of water falling from
a jet. You will now see that
it is at first a cylinder, that
as it goes down necks and
bulges begin to form, and at
last beads separate, and you
can see the little drops as well.
The beads also vibrate, be-
coming alternately long and
wide, and there can be no
doubt that the sparkling por-
tion of a jet, though it ap-
pears continuous, is really
made up of beads which pass
so rapidly before the eye that
it is impossible to follow
them. (I should explain that
for a reason which will ap-
pear later, I made a loud note
by whistling into a key at the
time that this photograph was taken.)
88
Lord Rayleigh has shown that in a stream
of water one twenty-fifth of an inch in diameter,
necks impressed upon the stream, even though
imperceptible, develop a thousandfold in depth
every fortieth of a second, and thus it is not
difficult to understand that in such a stream
the water is already broken through before it
has fallen many inches. He has also shown
that free water drops vibrate at a rate which
may be found as follows. A drop two inches
in diameter makes one complete vibration in
one second. If the diameter is reduced to one
quarter of its amount, the time of vibration
will be reduced to one-eighth, or if the diameter
is reduced to one-hundredth, the time will be
reduced to one-thousandth, and so on. The
same relation between the diameter and the
time of breaking up applies also to cylinders.
We can at once see how fast a bead of water
the size of one of those in the spider's web
would vibrate if pulled out of shape, and let
go suddenly. If we take the diameter as being
one eight-hundredth of an inch, and it is
really even finer, then the bead would have a
diameter of one sixteen-hundredth of a two-
inch bead, which makes one vibration in one
THE FORCES WHICH MOULD THEM. 89
second. It will therefore vibrate sixty-four
thousand times as fast, or sixty-four thousand
times a second. Water-drops the size of the
little beads, with a diameter of rather less than
one three-thousandth of an inch, would vibrate
half a million times a second, under the sole
influence of the feebly elastic skin of water !
We thus see how powerful is the influence of
the feebly elastic water-skin on drops of water
that are sufficiently small.
I shall now cause a small fountain to play,
and shall allow the water as it falls to patter
upon a sheet of paper. You can see both the
fountain itself and its shadow upon the screen.
You will notice that the water comes out of the
nozzle as a smooth cylinder, that it presently
begins to glitter, and that the separate drops
scatter over a great space (Fig. 41). Now why
should the drops scatter ? All the water comes
out of the jet at the same rate and starts in
the same direction, and yet after a short way the
separate drops by no means follow the same
paths. Now instead of explaining this, and
then showing experiments to test the truth of
the explanation, I shall reverse the usual order,
and show one or two experiments first, which
90 SOAP-BUBBLES, AND
I think you will all agree are so like magic, so
wonderful are they and yet so simple, that if
they had been performed a few hundred
years ago, the rash person who showed them
Fig. 41.
might have run a serious risk of being burnt
alive.
You now see the water of the jet scattering
in all directions, and you hear it making a
pattering sound on the paper on which it falls.
I take out of my pocket a stick of sealing-wax
and instantly all is changed, even though I am
THE FORCES WHICH MOULD THEM. 9!
some way off and can touch nothing. The
water ceases to scatter ; it travels in one con-
tinuous line (Fig. 42), and falls upon the paper
making a loud rattling noise which must re-
mind you of the rain of a thunder-storm.
Fig. 42.
I come a little nearer to the fountain and the
water scatters again, but this time in quite a
different way. The falling drops are much
larger than they were before. Directly I hide
the sealing-wax the jet of water recovers its old
92 SOAP-BUBBLES, AND
appearance, and as soon as the sealing-wax is
taken out it travels in a single line again.
Now instead of the sealing-wax I shall take
a smoky flame easily made by dipping some
cotton-wool on the end of a stick into benzine,
and lighting it. As long as the flame is held
away from the fountain it produces no effect,
but the instant that I bring it near so that the
water passes through the flame, the fountain
ceases to scatter ; it all runs in one line and falls
in a dirty black stream upon the paper. Ever
so little oil fed into the jet from a tube as
fine as a hair does exactly the same thing.
I shall now set a tuning-fork sounding at the
other side of the table. The fountain has not
altered in appearance. I now touch the stand
of the tuning-fork with a long stick which rests
against the nozzle. Again the water gathers
itself together even more perfectly than before,
and the paper upon which it falls is humming
out a note which is the same as that produced
by the tuning-fork. If I alter the rate at
which the water flows you will see that the
appearance is changed again, but it is never
like a jet which is not acted upon by a musical
sound. Sometimes the fountain breaks up
THE FORCES WHICH MOULD THEM.
93
into two or three and sometimes many more
distinct lines, as though it came out of as many
tubes of different sizes and pointing in slightly
different directions (Fig. 43). The effect of
different notes could be very easily shown if
any one were to sing to the piece of wood by
43-
which the jet is held. I can make noises of
different pitches, which for this purpose are
perhaps better than musical notes, and you
can see that with every new noise the fountain
puts on a different appearance. You may well
94 SOAP-BUBBLES.
wonder how these trifling influences sealing-
wax, the smoky flame, or the more or less
musical noise should produce this mysterious
result, but the explanation is not so difficult
as you might expect.
I hope to make this clear when we meet
again.
LECTURE III.
AT the conclusion of the last lecture I
showed you some curious experiments with a
fountain of water, which I have now to explain.
Consider what I have said about a liquid
cylinder. If it is a little more than three
times as long as it is wide, it cannot retain its
form ; if it is made very much more than three
times as long, it will break up into a series of
beads. Now, if in any way a series of necks
could be developed upon a cylinder which were
less than three diameters apart, some of them
would tend to heal up, because a piece of a
cylinder less than three diameters long is stable.
If they were about three diameters apart, the
form being "then unstable, the necks would get
more pronounced in time^ and would at last
break through, so that beads would be formed.
If necks were made at distances more than
three diameters apart, then the cylinder would
go on breaking up by the narrowing of these
96 SOAP-BUBBLES, AND
necks, and it would most easily break up into
drops when the necks were just four and a half
diameters apart. In other words, if a fountain
were to issue from a nozzle held perfectly still,
the water would most easily break into beads at
the distance of four and a half diameters apart,
but it would break up into a greater number
closer together, or a smaller number further
apart, if by slight disturbances of the jet very
slight waists were impressed upon the issuing
cylinder of water. When you make a fountain
play from a jet which you hold as still as
possible, there are still accidental tremors of all
kinds, which impress upon the issuing cylinder
slightly narrow and wide places at irregular dis-
tances, and so the cylinder breaks up irregu-
larly into drops of different sizes and at differ-
ent distances apart. Now these drops, as they
are in the act of separating from one another,
and are drawing out the waist, as you have
seen, are being pulled for the moment towards
one another by the elasticity of the skin of the
waist ; and, as they are free in the air to move
as they will, this will cause the hinder one to
hurry on, and the more forward one to lag
behind, so that unless they are all exactly
THE FORCES WHICH MOULD THEM. 97
alike both in size and distance apart they will
many of them bounce together before long.
You would expect when they hit one another
afterwards that they would join, but I shall be
able to show you in a moment that they do
not ; they act like two india-rubber balls, and
bounce away again. Now it is not difficult to
see that if you have a series of drops of differ-
ent sizes and at irregular distances bouncing
against one another frequently, they will tend
to separate and to fall, as we have seen, on all
parts of the paper down below. What did
the sealing-wax or the smoky flame do ? and
what can the musical sound do to stop this
from happening? Let me first take the
sealing-wax. A piece of sealing-wax rubbed
on your coat is electrified, and will attract light
bits of paper up to it. The sealing-wax acts
electrically on the different water-drops, causing
them to attract one another, feebly, it is true,
but with sufficient power where they meet to
make them break through the air-film between
them and join. To show that this is no fancy,
I have now in front of the lantern two foun-
tains of clean water coming from separate
bottles, and you can see that they bounce
9 8
apart perfectly (Fig. 44). To show that they
do really bounce, I have coloured the water in
the two bottles differently. The sealing-wax
is now in my pocket ; I shall retire to the other
side of the room, and the instant it appears
Fig. 44.
the jets of water coalesce (Fig. 45). This
may be repeated as often as you like, and
it never fails. These two bouncing jets are in
fact one of the most delicate tests for the pre-
sence of electricity that exist. You are now
able to understand the first experiment. The
THE FORCES WHICH MOULD THEM. 99
separate drops which bounced away from one
another, and scattered in all directions, are
unable to bounce when the sealing-wax is held
up, because of its electrical action. They
therefore unite, and the result is, that instead
Fig- 45-
of a great number of little drops falling all
over the paper, the stream pours in a single
line, and great drops, such as you see in a
thunder-storm, fall on the top of one another.
There can be no doubt that it is for this reason
that the drops of rain in a thunder-storm are
100
so large. This experiment and its explanation
are due to Lord Rayleigh.
The smoky flame, as lately shown by Mr.
Bidwell, does the same thing. The reason
probably is that the dirt breaks .through the
air-film, just as dust in the air will make the
two fountains join as they did when they were
electrified. However, it is possible that oily
matter condensed on the water may have some-
thing to do with the effect observed, because
oil alone acts quite as well as a flame, but the
action of oil in this case, as when it smooths
a stormy sea, is not by any means so easily
understood.
When I held the sealing-wax closer, the
drops coalesced in the same way ; but they
were then so much more electrified that they
repelled one another as similarly electrified
bodies are known -to do, and so the electrical
scattering was produced.
.You possibly already see why the tuning-
fork made the drops follow in one line, but
I shall explain. A musical note is, as is well
known, caused by a rapid vibration ; the more
rapid the vibration the higher is the pitch of
the note. For instance, I have a tooth-wheel
THE FORCES WHICH MOULD THEM. IOI
which I can turn round very rapidly if I wish.
Now that it is turning slowly you can hear
the separate teeth knocking against a card that
I am holding in the other hand. I am now
turning faster, and the card is giving out a
note of a low pitch. As I make the wheel
turn faster and faster, the pitch of the note
gradually rises, and it would, if I could only
turn fast enough, give so high a note that
we should not be able to hear it. A tuning-
fork vibrates at a certain definite rate, and
therefore gives a definite note. The fork now
sounding vibrates 128 times in every second.
The nozzle, therefore, is made to vibrate, but
almost imperceptibly, 128 times a second, and
to impress upon the issuing cylinder of water
128 imperceptible waists every second. Now
it just depends what size the jet is, and how
fast the water is issuing, whether these waists
are about four and a half diameters apart
in the cylinder. If the jet is larger, the water
must pass more quickly, or under a greater
pressure, for this to be the case ; if the jet is
finer, a smaller speed will be sufficient. If it
should happen that the waists so made are
anywhere, about four diameters apart, then
IO2 SOAP-BUBBLES, AND
even though they are so slightly developed
that if you had an exact drawing of them, you
would not be able to detect the slightest change
of diameter, they will grow at a great speed,
and therefore the water column will break up
regularly, every drop will be like the one
behind it, and like the one in front of it, and
not all different, as is the case when the break-
ing of the water merely depends upon acci-
dental tremors. If the drops then are all alike
in every respect, of course they all follow the
same path, and so appear to fall in a continuous
stream. If the waists are about four and a
half diameters apart, then the jet will break up
most easily ; but it will, as I have said, break
up under the influence of a considerable range
of notes, which cause the waists to be formed
at other distances, provided they are more
than three diameters apart. If two notes are
sounded at the same time, then very often
each will produce its own effect, and the result
is the alternate formation of drops of different
sizes, which then make the jet divide into two
separate streams. In this way, three, four, or
even many more distinct streams may be
produced.
THE FORCES WHICH MOULD THEM. 103
o a
a
o
(
o
%
t&
'-"' U
o a
Q
e
o
& o
o
O
O
O o
O
9
O
o
o
104 SOAP-RUBBLES, AND
I can now show you photographs of some
of these musical fountains, taken by the instan-
taneous flash of an electric spark, and you can
see the separate paths described by the drops
of different sizes (Fig. 46). In one photograph
there are eight distinct fountains all breaking
from the same jet, but following quite distinct
paths, each of which is clearly marked out by a
perfectly regular series of drops. You can also
in these photographs see drops actually in the
act of bouncing against one another, and flat-
tened when they meet, as if they were india-
rubber balls. In the photograph now upon
the screen the effect of this rebound, Which
occurs at the place marked with a cross, is to
hurry on the upper and more forward drop,
and to retard the other one, and so to make
them travel with slightly different velocities
and directions. It is for this reason that they
afterwards follow distinct paths. The smaller
drops had no doubt been acted on in a similar
way, but the part of the fountain where this
happened was just outside the photographic
plate, and so there is no record of what
occurred. The very little drops of which I
have so often spoken are generally thrown out
THE FORCES WHICH MOULD THEM. 105
from the side of a fountain of water under the
influence of a musical sound, after which they
describe regular little curves of their own, quite
distinct from the main stream. They, of
course, can only get out sideways after one or
two bouncings from the regular drops in front
and behind. You can easily show that they
are really formed below the place where they
first appear, by taking a piece of electrified
sealing-wax and holding it near the stream
close to the nozzle and gradually raising it.
When it comes opposite to the place where the
little drops are really formed, it will act on
them more powerfully than on the large drops,
and immediately pull them out from a place
where the moment before none seemed to
exist. They will then circulate in perfect
little orbits round the sealing-wax, just as the
planets do round the sun ; but in this case,
being met by the resistance of the air, the
orbits are spirals, and the little drops after
many revolutions ultimately fall upon the wax,
just as the planets would fall into the sun after
many revolutions, if their motion through space
were interfered with by friction of any kind.
There is only one thing needed to make the
IO6 SOAP-BUBBLES, AND
demonstration of the behaviour of a musical
jet complete, and that is, that you should your-
selves see these drops in their different posi-
tions in an actual fountain of water. Now if I
were to produce a powerful electric spark, then
it is true that some of you might for an instant
catch sight of the drops, but I do not think that
most would see anything at all. But if, instead
of making merely one flash, I were to make
another when each drop had just travelled to
the position which the one in front of it occu-
pied before, and then another when each drop
had moved on one place again, and so on, then
all the drops, at the moments that the flashes
of light fell upon them, would occupy the same
positions, and thus all these drops would appear
fixed in the air, though of course they really are
travelling fast enough. If, however, I do not
quite succeed in keeping exact time with my
flashes of light, then a curious appearance will
be produced. Suppose, for instance, that the
flashes of light follow one another rather too
quickly, then each drop will not have had quite
time enough to get to its proper place at each
flash, and thus at the second flash all the drops
will be seen in positions which are just behind
THE FORCES WHICH MOULD THEM. 107
those which they occupied at the first flash,
and in the same way at the third flash they
will be seen still further behind their former
places, and so on, and therefore they will
appear to be moving slowly backwards ; where-
as if my flashes do not follow quite quickly
enough, then the drops will, every time that
there is a flash, have travelled just a little too
far, and so they will all appear to be moving
slowly forwards. Now let us try the experi-
ment. There is the electric lantern sending a
powerful beam of light on to the screen. This
I bring to a focus with a lens, and then let it
pass through a small hole in a piece of card.
The light then spreads out and falls upon the
screen. The fountain of water is between the
card and the screen, and so a shadow is cast
which is conspicuous enough. Now I place
just behind the card a little electric motor,
which will make a disc of card which has six
holes near the edge spin round very fast.
The holes come one after the other opposite
the hole in the fixed card, and so at every
turn six flashes of light are produced. When
the card is turning about 21 J times a second,
then the flashes will follow one another at the
108 SOAP-BUBBLES, AND
right rate. I have now started the motor,
and after a moment or two I shall have
obtained the right speed, and this I know by
blowing through the holes, when a musical
note will be produced, higher than the fork if
the speed is too high, and lower than the
fork if the speed is too low, and exactly the
same as the fork if it is right.
To make it still more evident when the
speed is exactly right, I have placed the tuning-
fork also between the light and the screen, so
that you may see it illuminated, and its shadow
upon the screen. I have not yet allowed the
water to flow, but I want you to look at the
fork. For a moment I have stopped the
motor^ so that the light may be steady, and
you can see that the fork is in motion because
its legs appear blurred at the ends, where of
course the motion is most rapid. Now the
motor is started, and almost at once the fork
appears quite different. It now looks like
a piece of india-rubber, slowly opening and
shutting, and now it appears quite still, but the
noise it is making shows that it is not still by
any means. The legs of the fork are vibrating,
but the light only falls upon them at regular
THE FORCES WHICH MOULD THEM. 109
intervals, which correspond with their move-
ment, and so, as I explained in the case of the
water-drops, the fork appears perfectly still.
Now the speed is slightly altered, and, as I
have explained, each new flash of light, coming-
just too soon or just too late, shows the fork
in a position which is just before or just behind
that made visible by the previous flash. You
thus see the fork slowly going through its
evolutions, though of course in reality the legs
are moving backwards and forwards 128 times
a second. By looking at the fork or its
shadow, you will therefore be able to tell
whether the light is keeping exact time with
the vibrations, and therefore with the water-
drops.
Now the water is running, and you see all
the separate drops apparently stationary, strung
like pearls or beads of silver upon an invisible
wire (see Frontispiece). If I make the card
turn ever so little more slowly, then all the
drops will appear to slowly march onwards, and
what is so beautiful, but I am afraid few will
see this, each little drop may be seen to gradu-
ally break off', pulling out a waist which becomes
a little drop, and then when the main drop is
110 SOAP-BUBBLES, AND
free it slowly oscillates, becoming wide and long,
or turning over and over, as it goes on its way.
If it so happens that a double or multiple jet is
being produced, then you can see the little
drops moving up to one another, squeezing each
other where they meet and bouncing away
again. Now the card is turning a little too
fast and the drops appear to be moving back-
wards, so that it seems as if the water is coming
up out of the tank on the floor, quietly going
over my head, down into the nozzle, and
so back to the water-supply of the place.
Of course this is not happening at all, as you
know very well, and as you will see if I simply
try and place my finger between two of these
drops. The splashing of the water in all direc-
tions shows that it is not moving quite so
quietly as it appears. There is one more thing
that I would mention about this experiment.
Every time that the flashing light gains or
loses one complete flash, upon the motion of
the tuning-fork, it will appear to make one
complete oscillation, and the water-drops will
appear to move back or on one place.
I must now come to one of the most
beautiful applications of these musical jets
THE FORCES WHICH MOULD THEM. Ill
to practical purposes which it. is possible to
-imagine, and what I shall now show are a few
out of a great number of the experiments of
Mr. Chichester Bell, cousin of Mr. Graham
Bell, the inventor of the telephone.
To begin with I have a very small jet of
water forced through the nozzle at a great
pressure, as you can see if I point it towards
the ceiling, as the water rises eight or ten feet.
If I allow this stream of water to fall upon
an india-rubber sheet, stretched over the end
of a tube as big as my little finger, then the
little sheet will be depressed by the water, and
the more so if the stream is strong. Now
if I hold the jet close to the sheet the smooth
column of liquid will press the sheet steadily,
and it will remain quiet ; but if I gradually
take the jet further away from the sheet, then
any waists that may have been formed in the
liquid column, which grow as they travel, will
make their existence perfectly evident. When
a wide part of the column strikes the sheet it
will be depressed rather more than usual, and
when a narrow part follows, the depression will
be less. In other words, any very slight
vibration imparted to the jet will be magnified
I I 2 SOAP-BUBBLES, AND
by the growth of waists, and the sheet of india-
rubber will reproduce the vibration, but on a
magnified scale. Now if you remember that
sound consists of vibrations, then you will
understand that a jet is a machine for magnify-
ing sound. To show that this is the case I am
now directing the jet on to the sheet, and you
can hear nothing; but I shall hold a piece of
wood against the nozzle, and now, if on the
whole the jet tends to break up at any one rate
rather than at any other, or if the wood or the
sheet of rubber will vibrate at any rate most
easily, then the first few vibrations which cor-
respond to this rate will be 'imparted to the
wood, which will impress them upon the nozzle
and so upon the cylinder of liquid, where they
will become magnified ; the result is that the
jet immediately begins to sing of its own
accord, giving out a loud note (Fig. 47).
I will now remove the piece of wood. On
placing against the nozzle an ordinary lever
watch, the jolt which is imparted to the case
at every tick, though it is so small that you
cannot detect it, jolts the nozzle also, and thus
causes a neck to form in the jet of water which
will grow as it travels, and so produce a loud
THE FORCES WHICH MOULD THEM. 113
tick, audible in every part of this large room
(Fig. 48). Now I want you to notice how the
vibration is magnified by the action I have
described. I now hold the nozzle close to the
rubber sheet, and you can hear nothing. As I
Fig- 47
gradually raise it a faint echo is produced,
which gradually gets louder and louder, until
at last it is more like a hammer striking an
anvil than the tick of a watch.
I shall now change this watch for another
H
I 14 SOAP-BUBBLES, AND
which, thanks to a friend, I am able to use.
This watch is a repeater, that is, if you press
upon a nob it will strike, first the hour, then
the quarters, and then the minutes. I think the
Fig. 48.
water-jet will enable you all to hear what time
it|is. Listen ! one, two, three, four ; . , . ting-
tang, ting-tang; . . . one, two, three, four, five,
six. Six minutes after half-past four. You
notice that not only did you hear the number
of strokes, but the jet faithfully reproduced the
THE FORCES WHICH MOULD THEM. 115
musical notes, so that you could distinguish one
note from the others.
I can in the same way make the jet play a
tune by simply making the nozzle rest against
a long stick, which is pressed upon a musical-
box. The musical-box is carefully shut up in
a double box of thick felt, and you can hardly
hear anything ; but the moment that the nozzle
is made to rest against the stick and the water is
directed upon the india-rubber sheet, the sound
of the box is loudly heard, I hope, in every part
of the room. It is usual to describe a fountain
as playing, but it is now evident that a fountain
can even play a tune. There is, however, one
peculiarity which is perfectly evident. The jet
breaks up at certain rates more easily than at
others, or, in other words, it will respond to
certain sounds in preference to others. You
can hear that as the rnusical-box plays, certain
notes are emphasized in a curious way, pro-
ducing much the *same effect that follows if
you lay a penny upon the upper strings of a
horizontal piano.
Now, on returning to our soap-bubbles, you
may remember that I stated that the cate-
noid and the plane were the only figures of
Il6 SOAP-BUBBLES, AND
revolution which had no curvature, and which
therefore produced no pressure. There are
plenty of other surfaces which are apparently
curved in all directions and yet have no curva-
ture, and which therefore produce no pressure ;
but these are not figures of revolution, that is,
they cannot be obtained by
simply spinning a curved
line about an axis. These
may be produced in any
quantity by making wire
frames of various shapes
and dipping them in soap
and water. On taking them
out a wonderful variety of
surfaces of no curvature will
be seen. One such surface
is that known as the screw-
Fig. 49- surface. To produce this it
is only necessary to take a piece of wire wound
a .few times in an open helix (commonly called
spiral), and to bend the two ends so as to meet
a second wire passing down the centre. The
screw-surface developed by clipping this frame
in soap-water is well worth seeing (Fig. 49).
It is impossible to give any idea of the per-
THE FORCES WHICH MOULD THEM. 1IJ
fection of the form in a figure, but fortunately
this is an experiment which any one can easily
perform.
Then again, if a wire frame is made in the
shape of the edges of any of the regular
geometrical solids, very beautiful figures will
Fig. 50-
be found upon them after they have been
dipped in soap-water. In the case of the
triangular prism these surfaces are all flat, and
at the edges where these planes meet one
another there are always three meeting each
other at equal angles (Fig. 50). This, owing
to the fact that the frame is three-sided, is"
n8
not surprising. After looking at this three-
sided frame with three films meeting down
the central line, you might expect that with
a four-sided or square frame there would be
four films meeting each other in a line down
the middle. But it is a curious thing that it
does not matter how irregular the frame may
be, or how complicated a mass of froth may
be, there can never be more than three films
meeting in an edge, or more than four edges,
or six films, meeting in a point. Moreover
the films and edges can only meet one another
at equal angles. If for a moment by any
accident four films do meet in the same edge,
or if the angles are not exactly equal, then
the form, whatever it may be, is unstable ;
it cannot last, but the films slide over one
another and never rest until they have settled
down into a position in which the conditions of
stability are fulfilled. This may be illustrated
by a very simple experiment which you can
easily try at home, and which you can now
see projected upon the screen. There are two
pieces of window-glass about half an inch apart,
which form the sides of a sort of box into
which some soap and water have been poured.
THE FORCES WHICH MOULD THEM.
On blowing through a pipe which is immersed
in the water, a great number of bubbles are
formed between the plates. If the bubbles are
all large enough to reach across from one plate
to the other, you will at once see that there
are nowhere more than three films meeting
one another, and where they meet the angles
are all equal. The curvature of the bubbles
makes it difficult to see at first that the angles
really are all alike, but if you only look at a
very short piece close to where they meet, and
so avoid being bewildered by the curvature,
you will see that what I have said is true.
You will also see, if you are quick, that when
the bubbles are blown, sometimes four for a
moment do meet, but that then the films at
once slide over one another and settle down into
their only possible position of rest (Fig. 51).
The air inside a bubble is generally under
pressure, which is produced by its elasticity
and curvature. If the bubble would let the
air pass through it from one side to the other
of course it would soon shut up, as it did when
a ring was hung upon one, and the film within
the ring was broken. But there are no holes
in a bubble, and so you would expect that a
I2O SOAP-BUBBLES, AND
gas like air could not pass through to the
other side. Nevertheless it is a fact that gases
can slowly get through to the other side, and in
the case of certain vapours the process is far
more rapid than any one would think possible.
Ether produces a vapour which is very heavy,,
Fig- 5'-
and which also burns very easily. This vapour
can get to the other side of a bubble almost
at once. I shall pour a little ether upon blot-
ting-paper in this bell jar, and fill the jar with
its heavy vapour. You can see that the jar
is filled with something, not by looking at it,
for it appears empty, but by looking at its
THE FORCES WHICH MOULD THEM. 121
shadow on the screen. Now I tilt it gently
to one side, and you see something pouring
out of it, which is the vapour of ether. It is
easy to show that this is heavy ; it is only
necessary to drop into the jar a bubble, and
so soon as the bubble meets the heavy vapour
it stops falling and remains floating upon the
surface as a cork does
upon water (Fig. 52).
Now let me test the
bubble and see whether
any of the vapour has
passed to the inside. I
pick it up out of the jar
with a wire ring and carry
it to a light, and at once
there is a burst of flame.
But this is not sufficient
to show that the ether
vapour has passed to the inside, because it
might have condensed in sufficient quantity
upon the bubble to make it inflammable.
You remember that when I poured some of
this vapour upon water in the first lecture,
sufficient condensed to so weaken the water-
skin that the frame of wire could get through
Fig. 52-
122
SOAP-BUBBLES, AND
to the other side. However, I can see whether
this is the true explanation or not by blow-
ing a bubble on a wide pipe, and holding
it in the vapour for a moment. Now on
removing it you notice that the bubble hangs
Fig- 53-
like a heavy drop; it has lost the perfect
roundness that it had at first, and this looks as
if the vapour had found its way in, but this
is made certain by bringing a light to the
mouth of the tube, when the vapour, forced
THE FORCES WHICH MOULD THEM. 123
out by the elasticity of the bubble, catches
fire and burns with a flame five or six inches
long (Fig. 53). You might also have noticed
that when the bubble was removed, the vapour
inside it began to pass out again and fell
away in a heavy stream, but this you could
only see by looking at the shadow upon the
screen.
You may have noticed when I made the
drops of oil in the mixture of alcohol and
water, that when they were brought together
they did not at once unite ; they pressed against
one another and pushed each other away if
allowed, just as the water-drops did in the
fountain of which I showed you a photograph.
You also may have noticed that the drops of
water in the paraffin mixture bounced against
one another, or if filled with the paraffin, formed
bubbles in which often other small drops, both
of water and paraffin, remained floating.
In all these cases there was a thin film of
something between the drops which they were
unable to squeeze out, namely, water, paraffin,
or air, as the case might be. Will two soap-
bubbles also when knocked together be unable
to squeeze out the air between them ? This
124 SOAP-BUBBLES, AND
you can try at home just as well as I can here,
but I will perform the experiment at once. I
have blown a pair of bubbles, and now when
I hit them together they remain distinct and
separate (Fig. 54).
I shall next place a bubble on a ring, which
it is just too large to get through. In my
hand I hold a ring, on which I have a flat
Fig. 54-
film, made by placing a bubble upon it and
breaking it on one side. If I gently press
the bubble with the flat film, I can push it
through the ring to the other side (Fig. 55),
and yet the two have not really touched one
another at all. The bubble can be pushed
backwards and forwards in this way many
times.
I have now blown a bubble and hung it
below a ring. To this bubble I can hang
THE FORCES WHICH MOULD THEM.
another ring of thin wire, which pulls it a little
out of shape. Since the pressure inside is less
than that corresponding to a complete sphere,
and since it is greater than that outside, and this
we can tell by looking at the caps, the curve is
Fig- 55-
part of one of those represented by the dotted
lines in C or E, Fig. 31., However, without
considering the curve any more, I shall push
the end of the pipe inside, and blow another
bubble there, and let it go. It falls gently
1 26
SOAP-BUBBLES, AND
Fig. 56.
until it rests upon the outer bubble; not at
the bottom, because the heavy ring keeps that
part out of reach, but along
a circular line higher up
(Fig. 56). I can now drain
away the heavy drops of
liquid from below the
bubbles with a pipe, and
leave them clean and
smooth all over. I can
now pull the lower ring
down, squeezing the inner bubble into a shape
like an egg (Fig. 57), or swing it round and
round, and then with a little
care peel away the ring
from off the bubble, and
leave them both perfectly
round every way (Fig. 58).
I can draw out the air from
the outer bubble till you
can hardly see between
them, and then blow in,
and the harder I blow, the
more is it evident that the
two bubbles are not touching at all; the
inner one is now spinning round and round
(UNI
THE FORCES WHICII^MOULD THEM. 127
in the very centre of the large bubble, and
finally, on breaking the outer one the inner
floats away, none the worse for its very
unusual treatment.
There is a pretty variation of the last experi-
ment, which, however, requires that a little
green dye called fluorescine, or better, uranine,
should be dissolved in a separate dish of the
soap-water. Then you
can blow the outer
bubble with clean
soap-water, and the
inner one with the
coloured water. Then
if you look at the
* . Fl g- 5 8 -
two bubbles by ordin-
ary light, you will hardly notice any difference;
but if you allow sunlight, or electric light from
an arc lamp, to shine upon them, the inner one
will appear a brilliant green, while the outer
one will remain clear as before. They will not
mix at all, showing that though the inner one
is apparently resting against the outer one,
there is in reality a thin cushion of air between.
Now you know that coal-gas is lighter than
air, and so a soap-bubble blown with gas,
128
when let go, floats up to the ceiling at once.
I shall blow a bubble on a ring with coal-gas.
It is soon evident that it is pulling upwards.
I shall go on feeding it with gas, and I want
you to notice the very beautiful shapes that
it takes (Fig. 59, but imagine the globe inside
removed). These are all exactly the curves
that a water-drop assumes when hanging from
a pipe, except that they
are the other way up.
The strength of the skin
is now barely able to
withstand the pull, and
now the bubble breaks
away just as the drop of
water did.
I shall next place a bubble blown with air
upon a ring, and blow inside it a bubble
blown with a mixture of air and gas. It of
course floats up and rests against the top of
the outer bubble (Fig. 60). Now I shall let
a little gas into the outer one, until the sur-
rounding gas is about as heavy as the inner
bubble. It now no longer rests against the
top, but floats about in the centre of the large
bubble (Fig. 61), just as the drop of oil did
THE FORCES WHICH MOULD THEM.
129
Fig. 60.
in the mixture of alcohol and water. You
can see that the inner bubble is really lighter
than air, because if I
break the outer one, the
inner one rises rapidly to
the ceiling.
Instead of blowing the
first bubble on a heavy
fixed ring, I shall now
blow one on a light ring,
made of very thin wire. This bubble con-
tains only air. If I blow inside this a bubble
with coal-gas, then the gas-bubble will try
and rise, and will press
against the top of the
outer one with such force
as to make it carry up
the wire ring and a yard
of cotton, and some
paper to which the cotton
is tied (Fig. 62) ; and all
this time, though it is the inner one only
which tends to rise, the two bubbles are not
really touching one another at all.
I have now blown an air-bubble on the
fixed ring, and pushed up inside it a wire
Fig. 61.
13 SOAP-BUBBLES, AND
with a ring on the end. I shall now blow
another air-bubble on this inner ring. The
next bubble that I shall blow is
one containing gas, and this is in-
side the other two, and when let
go it rests against the top of the
second bubble. I next make the
second bubble a little lighter by
blowing a little gas into it, and
then make the outer one larger
with air. I can now peel off the
inner ring and take it away, leav-
ing the two inner bubbles free,
inside the outer one (Fig. 63).
And now the multiple reflections
of the brilliant colours of the dif-
ferent bubbles from one to the
other, set off by the beautiful
forms which the bubbles them-
Fig. 62. selves assume, give to the whole a
degree of symmetry and splendour
which you may go far to see equalled in any
other way. I have only to blow a fourth
bubble in real contact with the outer bubble
and the ring, to enable it to peel off and float
away with the other two inside.
Fig. 63.
THE FORCES WHICH MOULD THEM. 131
We have seen that bubbles and drops be-
have in very much the same way. Let us
see if electricity will
produce the same
effect that it did on
drops. You re-
member that a piece
of electrified sealing-
wax prevented a
fountain of water
from scattering, be-
cause where two
drops met, instead
of bouncing, they joined together. Now there
are on these two rings bubbles which are just
resting against one another, but not really
touching (Fig. 64). The
instant that I take out the
sealing-wax you see they
join together and become
one (Fig. 65). Two soap-
bubbles, therefore, enable
us to detect electricity,
even when present in minute quantity, just as
two water fountains did.
We can use a pair of bubbles to prove the
Fig. 64.
132 SOAP-BUBBLES, AND
truth of one of the well-known actions of
electricity. Inside an electrical conductor it is
impossible to feel any influence of electricity
outside, however much there may be, or how-
ever near you go to the surface. Let us,
therefore, take the two bubbles shown in Fig.
56, and bring an electrified stick of sealing-
Fig. 65.
wax near. The outer bubble is a conductor;
there is, therefore, no electrical action inside,
and this you can see because, though the
sealing-wax is so near the bubble that it pulls
it all to one side, and though the inner one
is so close to the outer one that you cannot
see between them, yet the two bubbles remain
THE FORCES WHICH MOULD THEM.
133
separate. Had there been the slightest elec-
trical influence inside, even to a depth of a
hundred-thousandth of an inch, the two
bubbles would have instantly come together.
There is one more experiment which I
must show, arid this will be the last; it is
Fig. 66.
a combination of the last two, and it beau-
tifully shows the difference between an in-
side and an outside bubble. I have now
a plain bubble resting against the side of the
pair that I have just been using. The instant
that I take out the sealing-wax the two outer
134 SOAP-BUBBLES.
bubbles join, while the inner one unharmed
and the heavy ring slide down to the bottom
of the now single outer bubble (Fig. 66).
And now that our time has drawn to a close
I must ask you whether that admiration and
wonder which we all feel when we play with
soap-bubbles has been destroyed by these
lectures ; or whether now that you know more
about them it is not increased. I hope you
will all agree with me that the actions upon
which such common and every-day phenomena
as drops and bubbles depend, actions which
have occupied the attention of the greatest
philosophers from the time of Newton to the
present day, are not so trivial as to be un-
worthy of the attention of ordinary people like
ourselves.
PRACTICAL HINTS.
I HOPE that the following practical hints
may be found useful by those who wish them-
selves to successfully perform the experiments
already described.
Drop with India-rubber Surface.
A sheet of thin india-rubber, about the
thickness of that used in air- balls, as it appears
before they have been blown out, must be
stretched over a ring of wood or metal eighteen
inches in diameter, and securely wired round
the edge. The wire will hold the india-rubber
better if the edge is grooved. This does not
succeed if tried on a smaller scale. This ex-
periment was shown by Sir W. Thomson at
the Royal Institution.
Jumping Frame.
This is easily made by taking a light glass
globe about two inches in diameter, such, for
136 SOAP-BUBBLES, AND
instance, as a silvered ball used to ornament a
Christmas-tree or the bulb of a pipette, which
is what I used. Pass through the open necks
of the bulb a piece of wire about one-twentieth
of an inch in diameter, and fix it permanently
and water-tight upon the wire by working into
the necks melted sealing-wax. An inch or
two above the globe, fasten a flat frame of thin
wire by soldering, or if this is too difficult, by
tying and sealing-wax. A lump of lead must
then be fastened or hung on to the lower end,
and gradually scraped away until the wire
frame will just be unable to force its way
through the surface of the water. None of
the dimensions or materials mentioned are of
importance.
Paraffined Sieve.
Obtain a piece of copper wire gauze with
about twenty wires to the inch, and cut out
from it a round piece about eight inches in
diameter. Lay it on a round block, of such a
size that it projects about one inch all round.
Then gently go round and round with the
hands pressing the edge down and keeping it
THE FORCES WHICH MOULD THEM. 137
flat above, until the sides are evenly turned down
all round. This is quite easy, because the wires
can allow of the kind of distortion necessary.
Then wind round the turned-up edge a few
turns of thick wire to make the sides stiff.
This ought to be soldered in position, but pro-
bably careful wiring will be good enough.
Melt some paraffin wax or one or two paraffin
candles of the best quality in a clean flat dish,
not over the fire, which would be dangerous,
but on a hot plate. When melted and clear
like water, dip the sieve in, and when all is hot
quickly take it out and knock it once or twice
on the table to shake the paraffin out of the
holes. Leave upside down until cold, and
then be careful not to scratch or rub off the
paraffin. This had best be done in a place
where a mess is of no consequence.
There is no difficulty in filling it or in setting
it to float upon water.
Narroiv Tubes and Capillarity.
Get some quill-glass tube from a chemist,
that is, tube about the size of a pen. If it is
more than, say, a foot long, cut off a piece by
138 SOAP-BUBBLES, AND
first making a firm scratch in one place with
a three-cornered file, when it will break at the
place easily. To make very narrow tube from
this, hold it near the ends in the two hands
very lightly, so that the middle part is high up
in the brightest part of an ordinary bright and
flat gas flame. Keep it turning until at last
it becomes so soft that it is difficult to hold it
straight. It can then be bent into any shape,
but if it is wanted to be drawn out it must be
held still longer until the black smoke upon
it begins to crack and peel up. Then quickly
take it out of the flame, and pull the two ends
apart, when a long narrow tube will be formed
.between. This can be made finer or coarser by
regulating the heat and the manner in which it
is pulled out. No directions will tell any one
so much as a very little practice. For drawing
out tubes the flame of a Bunsen burner or of
a blow-pipe is more convenient ; but for bend-
ing tubes nothing is so good as the flat gas
flame. Do not clean off smoke till the tubes
are cold, and do not hurry their cooling by
wetting or blowing upon them. In the country
where gas is not to be had, the flame of a
large spirit-lamp can be made to do, but it
THE FORCES WHICH MOULD THEM. 139
is not so good as a gas-flame. The narrower
these tubes are, the higher will clean water be
observed to rise in them. To colour the
water, paints from a cclour-box must not be
used. They are not liquid, and will clog the
very fine tubes. Some dye that will quite
dissolve (as sugar does) must be used. An
aniline dye, called soluble blue, does very well.
A little vinegar added may make the colour
last better.
Capillarity between Plates.
Two plates of flat glass, say three to five
inches square, are required. Provided they
are quite clean and well wetted there is no
difficulty. A little soap and hot water will
probably be sufficient to clean them.
Tears of Wine.
These are best seen at dessert in a glass
about half filled with port. A mixture of
from two to three parts of water, and one part
of spirits of wine containing a very little rosani-
line (a red aniline dye), to give it a nice colour,
may be used, if port is not available. A piece
I4O SOAP-BUBBLES, AND
of the dye about as large as a mustard-seed
will be enough for a large wine-glass. The
sides of the glass should be wetted with the
wine.
Cat-Boxes.
Every school-boy knows how to make these.
They are not the boxes made by cutting slits
in paper. They are simply made by folding,
and are then blown out like the " frog," which
is also made of folded paper.
Liquid Beads.
Instead of melting gold, water rolled on to
a table thickly dusted with lycopodium, or
other fine dust, or quicksilver rolled or thrown
upon a smooth table, will show the difference
in the shape of large and small beads perfectly.
A magnifying-glass will make the difference
more evident. In using quicksilver, be care-
ful that none of it falls on gold or silver coins,
or jewellery, or plate, or on the ornamental
gilding on book-covers. It will do serious
damage.
THE FORCES WHICH MOULD THEM. 141
Plateaus Experiment.
To perform this with very great perfection
requires much care and trouble. It is easy
to succeed up to a certain point. Pour into
a clean bottle about a table-spoonful of salad-
oil, and pour upon it a mixture of nine parts
by volume spirits of wine (not methylated
spirits), and seven parts of water. Shake up
and leave for a day if necessary, when it will
be found that the oil has settled together by
itself. Fill a tumbler with the same mixture
of spirit and water, and then with a fine glass
pipe, dipping about half-way down, slowly intro-
duce a very little water. This will make the
liquid below a little heavier. Dip into the oil a
pipe and take out a little by closing the upper
end with the finger, and carefully drop this into
the tumbler. If it goes to the bottom, a little
more water is required in the lower half of
the tumbler. If by chance it will not sink
at all, a little more spirit is wanted in the
upper half. At last the oil will just float in
the middle of the mixture. More can then
be added, taking care to prevent it from touch-
ing the sides. If the liquid below is ever so
IJ.2 SOAP-BUBBLES, AND
little heavier, and the liquid above ever so
little lighter than oil, the drop of oil perhaps
as large as a halfpenny will be almost per-
fectly round. It will not appear round if seen
through the glass, because the glass magnifies
it sideways, but not up and down, as may be
seen by holding a coin in the liquid just above
it. To see the drop in its true shape the vessel
must either be a globe, or one side must be
made of flat glass.
Spinning the oil so as to throw off a ring
is not material, but if the reader can contrive
to fix a disc about the size of a threepenny-
piece upon a straight wire, arid spin it round
without shaking it, then he will see the ring
break off, and either return if the rotation is
quickly stopped, or else break up into three or
four perfect little balls. The disc should be
wetted with oil before being dipped into the
mixture of spirit and water.
A Good Mixture for Soap- Bubbles.
Common yellow soap is far better than
most of the fancy soaps, which generally con-
tain a little soap and a lot of rubbish. Castille
THE FORCES WHICH MOULD THEM. 143
soap is very good, and this may be obtained
from any chemist.
Bubbles blown with soap and water alone
do not last long enough for many of the
experiments described, though they may some-
times be made to succeed. Plateau added
glycerine, which greatly improves the lasting
quality. The glycerine should be pure ; com-
mon glycerine is not good, but Price's answers
perfectly. The water should be pure distilled
water, but if this is not available, clean rain-
water will do. Do not choose the first that runs
from a roof after a spell of dry weather, but
wait till it has rained for some time, the water
that then runs off is very good, especially if
the roof is blue slate or glass. If fresh rain-
water is not to be had, the softest water should
be employed that can be obtained. Instead
of Castille soap, Plateau found that a pure
soap prepared from olive-oil is still better.
This is called oleate of soda. It should be
obtained freshly prepared from a manufactur-
ing chemist. Old, dry stuff that has been
kept a long time is not so good. I have
always used a modification of Plateau's for-
mula, which Professors Reinold and Riicker
144
found to answer so well. They used less
glycerine than Plateau. It is best made as
follows. Fill a clean stoppered bottle three-
quarters full of water. Add one-fortieth part of
its weight of oleate of soda, which will probably
float on the water. Leave it for a day, when
the oleate of soda will be dissolved. Nearly
fill up the bottle with Price's glycerine and
shake well, or pour it into another clean bottle
and back again several times. Leave the
bottle, stoppered of course, for about a week
in a dark place. Then with a syphon, that is,
a bent glass tube which will reach to the
bottom inside and still further outside, draw off
the clear liquid from the scum which will have
collected at the top. Add one or two drops
of strong liquid ammonia to every pint of the
liquid. Then carefully keep it in a stoppered
bottle in a dark place. Do not get out this
stock bottle every time a bubble is to be blown,
but have a small working bottle. Never put
any back into the stock. In making the liquid
do not warm or filter it. Either will spoil it.
Never leave the stoppers out of the bottles or
allow the liquid to be exposed to the air more
than is necessary. This liquid is still perfectly
THE FORCES WHICH MOULD THEM. 145
good after two years' keeping. I have given
these directions very fully, not because I feel
sure that all the details are essential, but
because it exactly describes the way I happen
to make it, and because I have never found
any other solution so good. Castille soap,
Price's glycerine, and rain-water will almost
certainly answer every purpose, and the same
proportions will probably be found to work
well.
Rings for Bubbles.
These may be made of any kind of wire.
I have used tinned iron about one-twentieth
of an inch in diameter. The joint should be
smoothly soldered without lumps. If solder-
ing is a difficulty, then use the thinnest wire
that is stiff enough to support the bubbles
steadily, and make the joint by twisting the
end of the wire round two or three times.
Rings two inches in diameter are convenient.
I have seen that dipping the rings in melted
paraffin is recommended, but I have not found
any advantage from this. The nicest material
for the light rings is thin aluminium wire,
about as thick as a fine pin (No. 26 to 30,
K
146 SOAP-BUBBLES, AND
B. W. G.), and as this cannot be soldered, the
ends must be twisted. If this is not to be
had, very fine wire, nearly as fine as a hair
(No. 36, B. W. G.), of copper or of any other
metal, will answer. The rings should be wetted
with the soap mixture before a bubble is placed
upon them, and must always be well washed
and dried when done with.
Threads in Ring.
There is no difficulty in showing these
experiments. The ring with the thread may
be dipped in the soap solution, or stroked
across with the edge of a piece of paper or
india-rubber sheet that has been dipped in the
liquid, so as to form a film on both sides of
the thread. A needle that has also been
wetted with the soap may be used to show
that the threads are loose. The same needle
held for a moment in a candle-flame supplies
a convenient means of breaking the film.
Blow out Candle with Soap-Bubble.
For this, the bubble should be blown on
the end of a short wide pipe, spread out at
one end to give a better hold for the bubble.
THE FORCES WHICH MOULD THEM. 147
The tin funnel supplied with an ordinary
gazogene answers perfectly. This should be
washed before it is used again for filling the
gazogene.
Bubbles balanced against one another.
These experiments are most conveniently
made on a small scale. Pieces of trun brass
tube, three-eighths or half an inch in diameter,
are suitable. It is best to have pieces of
apparatus, specially prepared with taps, for
easily and quickly stopping the air from leav-
ing either bubble, and for putting the two
bubbles into communication when required.
It should not be difficult to contrive to per-
form the experiments, using india-rubber con-
necting tubes, pinched with spring clips to
take the place of taps. There is one little
detail which just makes the difference between
success and failure. This is to supply a
mouth-piece for blowing the bubble, made of
glass tube, which has been drawn out so fine
that these little bubbles cannot be blown out
suddenly by accident. It is very difficult,
otherwise, to adjust the quantity of air in such
small bubbles with any accuracy. In balancing
148 SOAP-BUBBLES, AND
a spherical against a cylindrical bubble, the
short piece of tube, into which the air is sup-
plied, must be made so that it can be easily
moved to or from a fixed piece of the same
size closed at the other end. Then the two
ends of the short tube must have a film spread
over them with a piece of paper, or india-
rubber, but there must be no film stretched
across the end of the fixed tube. The two
tubes must at first be near together, until the
spherical bubble has been formed. They may
then be separated gradually more and more,
and air blown in so as to keep the sides of
the cylinder straight, until the cylinder is suf-
ficiently long to be nearly unstable. It will
then far more evidently show, by its change of
form, than it would if it were short, when the
pressure due to the spherical bubble exactly
balances that due to a cylindrical one. If the
shadow of the bubbles, or an image formed
by a lens on a screen, is then measured, it will
be found that the sphere has a diameter which
is very accurately double that of the cylinder.
THE FORCES WHICH MOULD THEM. 149
Thaumatrope for shoiving the Formation and
Oscillations of Drops.
The experiment showing the formation of
water-drops can be very perfectly imitated,
and the movements actually made visible, with-
out any necessity for using liquids at all, by
simply converting Fig. 35 (at end of book) into
the old-fashioned instrument called a thauma-
trope. What will then be seen is a true repre-
sentation, because the forms in the figure are
copies of a series of photographs taken from
the moving drops at the rate of forty-three
photographs in two seconds. 1
Obtain a piece of good cardboard as large
as the figure, and having brushed it all over
on one side with thin paste, lay the figure
upon it, and press it down evenly. Place it
upon a table, and cover it with a few thick-
nesses of blotting-paper, and lay over all a flat
piece of board large enough to cover it.
Weights sufficient to keep it all flat may be
added. This must be left all night at least,
until the card is quite dry, or else it will curl
i For particulars see Philosophical Magazine^ Sep-
tember 1890.
r 5
up and be useless. Now with a sharp chisel
or knife, but a chisel if possible, cut out the
forty-three slits near the edge, accurately
following the outline indicated in black and
white, and keeping the slits as narrow as
possible. Then cut a hole in the middle, so as
to fit the projecting part of a sewing-machine
cotton-reel, and fasten the cotton-reel on the
side away from the figure with glue or small
nails. It must be fixed exactly in the middle.
The edge should of course be cut down to
the outside of the black rim.
Now having found a pencil or other rod
on which the cotton-reel will freely turn, use
this as an axle, and holding the disc up in
front of a looking-glass, and in a good light,
slowly and steadily make it turn round. The
image of the disc seen through the slit in the
looking-glass will then perfectly represent every
feature of the growing and falling drop. As
the drop grows it will gradually become too
heavy to be supported, a waist will then begin
to form which will rapidly get narrower, until
the drop at last breaks away. It will be seen
to continue its fall until it has disappeared in the
liquid below, but it has not mixed with this,
THE FORCES WHICH MOULD THEM. 1^1
and so it will presently appear again, having
bounced out of the liquid. As it falls it will
be seen to vibrate as the result of the sudden
release from the one-sided pull. The neck
which was drawn out will meanwhile have
gathered itself in the form of a little drop, which
will then be violently hit by the oscillations of
the remaining pendant drop above, and driven
down. The pendant drop will be seen to
vibrate and grow at the same time, until it
again breaks away as before, and so the
phenomena are repeated.
In order to perfectly reproduce the experi-
ment, the axle should be firmly held upon a
stand, and the speed should not exceed one
turn in two seconds.
The effect is still more real if a screen is
placed between the disc and the mirror, which
will only allow one of the drops to be seen.
Water-drops in Paraffin and Bisulphide of
Carbon.
All that was said in describing the Plateau
experiment applies here. Perfectly spherical
and large drops of water can be formed in a
152, SOAP-BUBBLES, AND
mixture so made that the lower parts are very
little heavier, and the upper parts very little
lighter, than water. The addition of bisulphide
of carbon makes the mixture heavier. This
liquid bisulphide of carbon is very danger-
ous, and has a most dreadful smell, so that it
had better not be brought into the house. The
form of a hanging drop, and the way in which
it breaks off, can be seen if water is used in
paraffin alone, but it is much more evident
if a little bisulphide of carbon is mixed with
the paraffin, so that water will sink slowly
in the mixture. Pieces of glass tube, open
at both ends from half an inch to one inch
in diameter, show the action best. Having
poured some water coloured blue into a glass
vessel, and covered it to a depth of several
inches with paraffin, or the paraffin mixture,
dip the pipe down into the water, having first
closed the upper end with the thumb or the
palm of the hand. On then removing the
hand, the water will rush up inside the tube.
Again close the upper end as before, and raise
the tube until ths lower end is well above the
water, though still immersed in the paraffin.
Then allow air to enter the pipe very slowly
THE FORCES WHICH MOULD THEM. 1^3
by just rolling the thumb the least bit to
one side. The water will escape slowly and
form a large growing drop, the size of which,
before it breaks away, will depend on the
density of the mixture and the size of the tube.
To form a water cylinder in the paraffin the
tube must be filled with water as before, but
the upper end must now be left open. Then
when all is quiet the tube is to be rather
rapidly withdrawn in the direction of its own
length, when the water which was within it
will be left behind in form of a cylinder,
surrounded by the paraffin. It will then break
up into spheres so slowly, in the case of a
large tube, that the operation can be watched.
The depth of paraffin should be quite ten times
the diameter of the tube.
To make bubbles of water in the paraffin,
the tube must be dipped down into the water
with the upper end open all the time, so that
the tube is mostly filled with paraffin. It
must then be closed for a moment above and
raised till the end is completely out of the
water. Then if air is allowed to enter slowly,
and the tube is gently raised, bubbles of water
filled with paraffin will be formed which can
SOAP-BUBBLES, AND
be made to separate from the pipe, like soap-
bubbles from a " churchwarden," by a suitable
sudden movement. If a number of water-
drops are floating in the paraffin in the pipe,
and this can be easily arranged, then the
bubbles made will contain possibly a number
of other drops, or even other bubbles. A very
little bisulphide of carbon poured carefully
down a pipe will form a heavy layer above
the water, on which these compound bubbles
will remain floating.
Cylindrical bubbles of water in paraffin may
be made by dipping the pipe down into the
water and withdrawing it quickly without ever
closing the top at all. These break up into
spherical bubbles in the same way that the
cylinder of liquid broke up into spheres of
liquid.
Beaded Spider-webs.
These are found in the spiral part of the
webs of all the geometrical spiders. The
beautiful geometrical webs may be found out
of doors in abundance in the autumn, or in
green-houses at almost any time of the year.
To mount these webs so that the beads may
THE FORCES WHICH MOULD THEM. 155
be seen, take a small flat ring of any material,
or a piece of card-board with a hole cut out
with a gun-wad cutter, or otherwise. Smear
the face of the ring, or the card, with a very
little strong gum. Choose a freshly-made
web, and then pass the ring, or the card, across
the web so that some of the spiral web (not
the central part of the web) remains stretched
across the hole. This must be done without
touching or damaging the pieces that are
stretched across, except at their ends. The
beads are too small to be seen with the naked
eye. A strong magnifying-glass, or a low
power microscope, will show the beads and
their marvellous regularity. The beads on the
webs of very young spiders are not so regular
as those on spiders that are fully grown. Those
beautiful beads, easily visible to the naked eye,
on spider lines in the early morning of an
autumn day, are not made by the spider, but
are simply dew. They very perfectly show the
spherical form of small water-drops.
"56
Photographs of Water-jets.
These are easily taken by the method
described by Mr. Chichester Bell. The flash
of light is produced by a short spark from
a few Leyden-jars. The fountain, or jet, should
be five or six feet away from the spark, and
the photographic plate should be held as close
to the stream of water as is possible without
touching. The shadow is then so definite that
the photograph, when taken, may be examined
with a powerful lens, and will still appear sharp.
Any rapid dry plate will do. The room, of
course, must be quite dark when the plate is
placed in position, and the spark then made.
The regular breaking up of the jet may be
effected by sound produced in almost any way.
The straight jet, of which Fig. 41 is a repre-
sentation, magnified about three and a quarter
times, was regularly broken up by simply
whistling to it with a key. The fountains were
broken up regularly by fastening the nozzle to
one end of a long piece of wood clamped at
the end to the stand of a tuning-fork, which
was kept sounding by electrical means. An
ordinary tuning-fork, made to rest when sound-
THE FORCES WHICH MOULD THEM. 157
ing against the wooden support of the nozzle,
will answer quite as well, but is not quite so
convenient. The jet will break up best to
certain notes, but it may be tuned to a great
extent by altering the size of the orifice or
the pressure of the water, or both.
Fountain and Sealing-wax.
It is almost impossible to fail over this very
striking yet simple experiment. A fountain
of almost any size, at any rate between one-
fiftieth and a quarter of an inch in the smooth
part, and up to eight feet high, will cease to
scatter when the sealing-wax is rubbed with
flannel and held a few feet away. A suitable
size of fountain is one about four feet high,
coming from an orifice anywhere near one-
sixteenth of an inch in diameter. The nozzle
should be inclined so that the water falls
slightly on one side. The sealing-wax may be
electrified by being rubbed on the coat-sleeve,
or on a piece of fur or flannel which is dry. It
will then make little pieces of paper or cork
dance, but it will still act on the fountain when
158 SOAP-BUBBLES, AND
it has ceased to produce any visible effect on
pieces of paper, or even on a delicate gold-leaf
electroscope.
Bouncing Water-jets.
This beautiful experiment of Lord Ray-
Jeigh's requires a little management to make it
work in a satisfactory manner. Take a piece
of quill-glass tube and draw it out to a very
slight extent (see a former note), so as to
make a neck about one-eighth of an inch
in diameter at the narrowest part. Break the
tube just at this place, after first nicking it
there with a file. Connect each of these tubes
by means of an india-rubber pipe, or other-
wise, with a supply of water in a bottle, and
pinch the tubes with a screw-clip until two
equal jets of water are formed. So hold the
nozzles that these meet in their smooth por-
tions at every small angle. They will then
for a short time bounce away from one
another without mixing. If the air is very
dusty, if the water is not clean, or if air-
bubbles are carried along in the pipes, the
two jets will at once join together. In the
THE FORCES WHICH MOULD THEM. 159
arrangement that I used in the lantern, the
two nozzles were nearly horizontal, one was
about half an inch above the other, and they
were very slightly converging. They were
fastened in their position by melting upon
them a little sealing-wax. India-rubber pipes
connected them with two bottles about six
inches above them, and screw-clips were used
to regulate the supply. One of the bottles
was made to stand on three pieces of seal-
ing-wax to electrically insulate it, and the
corresponding nozzle was only held by its
sealing-wax fastening. The water in the bottles
had been filtered, and one was coloured blue.
If these precautions are taken, the jets will
remain distinct quite long enough, but are
instantly caused to recombine by a piece of
electrified sealing-wax six or eight feet away.
They may be separated again by touching the
water issuing near one nozzle with the finger,
which deflects it; on quietly removing the
finger the jet takes up its old position and
bounces off the other as before. They can
thus be separated and made to combine ten
or a dozen times in a minute.
i6o
Fountain and Intermittent Light.
This can be successfully shown to a large
number of people at once only by using an
electric arc, but there is no occasion to produce
this light if not more than one person at a time
wishes to see the evolution of the drops. It is
then merely necessary to make the fountain play
in front of a bright background such as the
sky, to break it up with a tuning-fork or other
musical sound as described, and then to look
at it through a card disc equally divided near
the edge into spaces about two or three inches
wide, with a hole about one-eighth of an inch
in diameter between each pair of spaces. A
disc of card five inches in diameter, with six
equidistant holes half an inch from the edge,
answers well. The disc must be made to
spin by any means very regularly at such
a speed that the tuning-fork, or stretched
string if this be used, when looked at through
the holes, appears quiet, or nearly quiet, when
made to vibrate. The separate drops will
then be seen, and everything described in the
preceding pages, and a great deal more, will
be evident. This is one of the most fascin-
THE FORCES WHICH MOULD THEM. l6l
ating experiments, and it is well worth while
to make an effort to succeed. The little
motor that I used is one of Cuttriss and Co.'s
P. i. motors, which are very convenient for
experiments of this kind. It was driven by
four Grove's cells. These make it rotate too
fast, but the speed can be reduced by moving
the brushes slightly towards the position used
for reversing the motor, until the speed is
almost exactly right. It is best to arrange
that it goes only just too fast, then the speed
can be perfectly regulated by a very light pres-
sure of the finger on the end of the axle.
Mr. Chichester Bell's Singing Water -jet.
For these experiments a very fine hole
about one seventy-fifth of an inch in diameter
is most suitable. To obtain this, Mr. Bell
holds the end of a quill-glass tube in a blow-
pipe flame, and constantly turns it round and
round until the end is almost entirely closed
up. He then suddenly and forcibly blows into
the pipe. Out of several nozzles made in this
way, some are sure to do well. Lord Rayleigh
makes nozzles generally by cementing to the
1 62, SOAP-BUBBLES, AND
end of a glass (or metal) pipe a piece of thin
sheet metal in which a hole of the required
size has been made. The water pressure should
be produced by a head of about fifteen feet.
The water must be quite free from dust and
from air-bubbles. This may be effected by
making it pass through a piece of tube stuffed
full of flannel, or cotton-wool, or something of
the kind to act as a filter. There should be
a yard or so of good black india-rubber tube,
about one-eighth of an inch in diameter inside,
between the filter and the nozzle. It is best
not to take the water direct from the water-
main, but from a cistern about fifteen feet
above the nozzle. If no cistern is available,
a pail of water taken up-stairs, with a pipe
coming down, is an excellent substitute, and
this has the further advantage that the head
of water can be easily changed so as to arrive
at the best result.
The rest of the apparatus is very simple.
It is merely necessary to stretch and tie over
the end of a tube about half an inch in
diameter a piece of thin india-rubber sheet,
cut from an air-ball that has not been blown
out. The tube, which may be of metal or of
THE FORCES WHICH MOULD THEM. 163
glass, may either be fastened to a heavy foot,
in which case a side tube must be joined to it,
as in Fig. 47, or it may be open at both ends
and be held in a clamp. It is well to put a
cone of card-board on the open end (Fig. 48),
if the sound is to be heard by many at a time.
If the experimenter alone wishes to hear as
well as possible when faint sounds are pro-
duced, he should carry a piece of smooth india-
rubber tube about half an inch in diameter
from the open end to his ear. This, however,
would nearly deafen him with such loud noises
as the tick of a watch.
Bubbles and Ether.
Experiments with ether must be performed
with great care, because, like the bisulphide of
carbon, it is dangerously inflammable. The
bottle of ether must never be brought near
a light. If a large quantity is spilled, the
heavy vapour is apt to run along the floor and
ignite at a fire, even on the other side of a
room. Any vessel may be filled with the
vapour of ether by merely pouring the liquid
upon a piece of blotting-paper reaching up to
164
the level of the edge. Very little is required,
say half a wine-glassful, for a basin that
would hold a gallon or more. In a draughty
place the vapour will be lost in a short time.
Bubbles can be set to float upon the vapour
without any difficulty. They may be removed
in five or ten seconds by means of one of the
small light rings with a handle, provided that
the ring is wetted with the soap solution and
has no film stretched across it. If taken to
a light at a safe distance the bubble will
immediately burst into a blaze. If a neigh-
bouring light is not close down to the table,
but well up above the jar on a stand, it may
be near with but little risk. To show the
burning vapour, the same wide tube that was
used to blow out the candle will answer well.
The pear shape of the bubble, owing to its
increased weight after being held in the vapour
for ten or fifteen seconds, is evident enough
on its removal, but the falling stream of heavy
vapour, which comes out again afterwards, can
only be shown if its shadow is cast upon a
screen by means of a bright light.
THE FORCES WHICH MOULD THEM. 165
Experiment with Internal Bubbles.
For these experiments, next to a good solu-
tion, the pipe is of the greatest importance.
A " churchwarden " is no use. A glass pipe
T 5 F inch in diameter at the mouth is best.
If this is merely a tube bent near the end
through a right angle, moisture condensed in
the tube will in time run down and destroy
the bubble occasionally, which is very annoy-
ing in a difficult experiment. I have made
for myself the pipe of which Fig. 67 is a full
size representation, and I do not think that
it is possible to improve upon this. Those
who are not glass-blowers will be able, with
the help of cork, to make a pipe with a trap
as shown in Fig. 68, which is as good, except
in appearance and handiness.
In knocking bubbles together to show that
they do not touch, care must be taken to
avoid letting either bubble meet any projection
in the other, such as the wire ring, or a heavy
drop of liquid. Either will instantly destroy
the two bubbles. There is also a limit to the
violence which may be used, which experience
will soon indicate.
i66
SOAP-BUBBLES, AND
In pushing a bubble
through a ring smaller than
itself, by means of a flat
film on another ring, it is
important that the bubble
should not be too large ; but
a larger bubble can be
pushed through than would
be expected. It is not so
easy to push it up as down
because of the 'heavy drop of
liquid, which it is difficult
to completely drain away.
To blow one bubble inside
another, the first, as large as
an average orange, should be
blown on the lower side of
a horizontal ring. A light
Fig. 67.
wire ring should then be hung on to this bubble
to slightly pull it out of shape. For this pur-
pose thin aluminium rings are hardly heavy
enough, and so either a heavier metal should
THE FORCES WHICH MOULD THEM.
,67
be used, or a small weight
should be fastened to the
handle of the ring. The
ring should be so heavy that
the sides of the bubble make
an angle of thirty or forty
degrees with the vertical,
where they meet the ring as
indicated in Fig. 56. The
wetted end of the pipe is now
to be inserted through the
top of the bubble, until it
has penetrated a clear half
inch or so. A new bubble
can now be blown any size
almost that may be desired.
Fig. 68.
To remove the pipe a slow motion will be
fatal, because it will raise the inner bubble
until it and the outer one both meet the pipe
at the same place. This will bring them into
true contact. On the other hand, a violent
I 68 SOAP-BUBBLES, AND
jerk will almost certainly produce too great a
disturbance. A rather rapid motion, or a
slight jerk, is all that is required. It is advis-
able before passing the pipe up through the
lower ring, so as to touch the inner bubble,
and so drain away the heavy drop, to steady
this with the other hand. The superfluous
liquid can then be drained from both bubbles
simultaneously. Care must be taken after
this that the inner bubble is not allowed to
come against either wire ring, nor must the
pipe be passed through the side where the two
bubbles are very close together. To peel off
the lower ring it should be pulled down a very
little way and then inclined to one side. The
peeling will then start more readily, but as
soon as it has begun the ring should be raised
so as not to make the peeling too rapid, other-
wise the final jerk, when it leaves the lower
ring, will be too much for the bubbles to
withstand.
Bubbles coloured with fluorescine, or uranine,
do not show their brilliant fluorescence unless
sunlight or electric light is concentrated upon
them with a lens or mirror. The quantity of
dye required is so small that it may be difficult
THE FORCES WHICH MOULD THEM. 169
to take little enough. As much as can be
picked up on the last eighth of an inch of
a pointed pen-knife will be, roughly speak-
ing, enough for a wine-glassful of the soap
solution. If the quantity is increased beyond
something like the proportion stated, the fluor-
escence becomes less and very soon disappears.
The best quantity can be found in a few
minutes by trial.
To blow bubbles containing either coal-gas
or air, or a mixture of the two, the most
convenient plan is to have a small T-shaped
glass tube which can be joined by one arm of
the T to the blow-pipe by means of a short
piece of india-rubber tube, and be connected
by its vertical limb with a sufficient length of
india-rubber pipe, one-eighth of an inch in
diameter inside, to reach to the floor, after
which it may be connected by any kind of
pipe with the gas supply. The gas can be
stopped either by pinching the india-rubber
tube with the left hand, if that is at liberty,
or by treading on it if both hands are occu-
pied. Meanwhile air can be blown in by the
other arm of the T, and the end closed by
the tongue when gas alone is required. This
170 SOAP-BUBBLES, AND
end of the tube should be slightly spread out
when hot by rapidly pushing into it the cold
tang of a file, and twisting it at the same time,
so that it may be lightly held by the teeth
without fear of slipping.
If a light T-piece or so great a length of
small india-rubber tube cannot be obtained,
then the mouth must be removed from the pipe
and the india-rubber tube slipped in when air
is to be changed for gas. This makes the
manipulation more difficult, but all the experi-
ments, except the one with three bubbles, can
be so carried out.
The pipe must in every case be made to
enter the highest point of a bubble in order
to start an internal one. If it is pushed
horizontally through the side, the inner bubble
is sure to break. If the inner bubble is being
blown with gas, it will soon tend to rise. The
pipe must then be turned over in such a
manner that the inner bubble does not creep
along it, and so meet the outer one where
penetrated by the pipe. A few trials will show
what is meant. The inner bubble may then
be allowed to rest against the top of the outer
one while being enlarged. When it is desired
THE FORCES WHICH MOULD THEM. 171
after withdrawing the pipe to blow more air
or gas into either the inner or the outer bubble,
it is not safe after inserting the pipe again to
begin to blow at once ; the film which is now
stretched across the mouth of the pipe will
probably become a third bubble, and this, under
the circumstances, is almost certain to cause a
failure, An instantaneous withdrawal of the air
destroys this film by drawing it into the pipe.
Air or gas may then be blown without danger.
If the same experiment is performed upon
a light ring with cotton and paper attached,
the left hand will be occupied in holding this
ring, and then the gas must be controlled by
the foot, or by a friend. The light ring-
should be quite two inches in diameter. If,
when the inner bubble has begun to carry
away the ring, &c., the paper is caught hold
of, it is possible, by a judicious pull, to cause
the two bubbles to leave the ring and so
escape into the air one inside the other. For
this purpose the smallest ring that will carry
the paper should be used. With larger rings
the same effect may be produced by inclining
the ring, and so allowing the outer bubble to
peel off, or by placing the mouth of the pipe
172
against the ring and blowing a third bubble
in real contact with the ring and the outer
bubble. This will assist the peeling process.
To blow three bubbles, one inside the other
two, is more difficult. The following plan I
have found to be fairly certain. First blow
above the ring a bubble the size of a large
orange. Then take a small ring about an inch
in diameter, with a straight wire coming down
from one side to act as a handle, and after
wetting it with the solution, pass it carefully
up through the fixed ring so that the small
ring is held well inside the bubble. Now
pass the pipe, freshly dipped in the solution,
into the outer or No. i bubble until it is
quite close to the small ring, and begin to
blow the No. 2 bubble. This must be started
with the pipe almost in contact with the inner
ring, as the film on this ring would destroy
a bubble that had attained any size. With-
draw the pipe, dip it into the liquid, and
insert it into the inner bubble, taking care
to keep these two bubbles from meeting any-
where. Now blow a large gas-bubble, which
may rest against the top of No. 2 while it is
growing. No. 2 may now rest against the
THE FORCES WHICH MOULD THEM. 173
top of No. i without danger. Remove pipe
from No. 3 by gently lowering it, and let
some gas into No. 2 to make it lighter, and
at the same time diminish the pressure between
Nos. 2 and 3. Presently the small ring can
be peeled off No. 2 and removed altogether.
But if there is a difficulty in accomplishing
this, withdraw the pipe from No. 2 and blow
air into No. i to enlarge it, which will make
the process easier. Then remove the pipe
from No. i. The three bubbles are now
resting one inside the other. By blowing a
fourth bubble, as described above, against the
fixed ring, No. i bubble will peel ofi] and the
three will float away. No. i can, while peel-
ing, be transferred to a light wire ring from
which paper, &c. are suspended. This de-
scription sounds complicated, but after a little
practice the process can be carried out almost
with certainty in far less time than it takes
to describe it; in fact, so quickly can it be
done, and so simple does it appear, that no one
would suppose that so many details had to be
attended to.
174 SOAP-BUBBLES, AND
Bubbles and Electricity.
These experiments are on the whole the
most difficult to perform successfully. The
following details should be sufficient to pre-
vent failure. Two rings are formed at the
end of a pair of wires about six inches long
in the straight part. About one inch at the
opposite end from the ring is turned down at
a right angle. These turned-down ends rest
in two holes drilled vertically in a non-
conductor such as ebonite, about two or
three inches apart. Then if all is right the
two rings are horizontal and at the same level,
and they may be moved towards or away from
one another. Separate them a few inches, and
blow a bubble above or below each, making
them nearly the same size. Then bring the
two rings nearer together until the bubbles
just, and only just, rest against one another.
Though they may be hammered together
without joining, they will not remain long
resting in this position, as the convex sur-
faces can readily squeeze out the air. The
THE FORCES WHICH 1VTOULD THEM. "1 75
ebonite should not be perfectly warm and dry,
for it is then sure to be electrified, and this
will give trouble. It must not be wet, because
then it will conduct, and the sealing-wax will
produce no result. If it has been used as
the support for the rings for some of the pre-
vious experiments, it will have been sufficiently
splashed by the bursting of bubbles to be in
the best condition. It must, however, be well
wiped occasionally.
A stick of sealing-wax should be held in
readiness under the arm, in a fold or two of
dry flannel or fur. If the wax is very strongly
electrified, it is apt to be far too powerful, and
to cause the bubbles, when it is presented to
them, to destroy each other. A feeble electri-
fication is sufficient ; then the instant it is
exposed the bubbles coalesce. The wax may
be brought so near one bubble in which
another one is resting, that it pulls them to
one side, but the inner one is screened from
electrical action by the outer one. It is im-
portant not to bring the wax very near, as in
that case the bubble will be pulled so far as
to touch it, and so be broken. The wetting
176 SOAP-BUBBLES, AND
of the wax will make further electrification very
uncertain. In showing the difference between
an inner and an outer bubble, the same re-
marks with regard to undue pressure, electrifi-
cation, or loss of time apply. I have generally
found that it is advisable in this experiment
not to drain the drops from both the bubbles,
as their weight seems to steady them ; the
external bubble may be drained, and if it is
not too large, the process of electrically join-
ing the outer bubbles, without injury to the
inner one, may be repeated many times. I
once caused eight or nine single bubbles to
unite with the outer one of a' pair in succes-
sion before it became too unwieldy for more
accessions to be possible.
It would _ be going outside my subject
to say anything about the management of
lanterns. I may, however, state that while
the experiments with the small bubbles are
best projected with a lens upon the screen, the
larger bubbles described in the last lecture can
only be projected by their shadows. For this
purpose the condensing lens is removed, and
THE FORCES WHICH MOULD THEM. 177
the bare light alone made use of. An electric
arc is far preferable to a lime-light, both
because the shadows are sharper, and because
the colours are so much more brilliant. No
oil lamp would answer, even if the light were
sufficient in quantity, because the flame would
be far too large to cast a sharp shadow.
In these hints, which have in themselves
required a rather formidable chapter, I have
given all the details, so far as I am able, which
a considerable experience has shown to be
necessary for the successful performance of:
the experiments in public. The hints will I
hope materially assist those who are not in the
habit of carrying out experiments, but who
may wish to perform them for their own satis-
faction. Though people who are not ex-
perimentalists may consider that the hints
are overburdened with detail, it is probable
that in repeating the experiments they will
find here and there, in spite of all my care
to provide against unforeseen difficulties, that
more detail would have been desirable.
Though it is unusual to conclude such a
book as this with the fullest directions for
M
178 SOAP-BUBBLES.
carrying out the experiments described, I
believe that the innovation in the present
instance is good, more especially because many
of the experiments require none of the elabor-
ate apparatus which so often is necessary.
OF THE
XJHIVERSIT
THE END.
Richard Clay &> So/is, Limited, London & Bungny.
LOAN DEPT
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