one-twentieth of an inch in diameter. The joint should be smoothly
soldered without lumps. If soldering 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, 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 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
thin 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 leaving either
bubble, and for putting the two bubbles into communication when
required. It should not be difficult to contrive to perform the
experiments, using india-rubber connecting 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 a spherical
against a cylindrical bubble, the short piece of tube, into which the
air is supplied, 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 sufficiently 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.
_Thaumatrope for showing 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, without any
necessity for using liquids at all, by simply converting Fig. 35 (at end
of book) into the old-fashioned instrument called a thaumatrope. What
will then be seen is a true representation, 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.[2]
[Footnote 2: For particulars see _Philosophical Magazine_, September
1890.]
Obtain a piece of good card-board 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
thicknesses 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 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, 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 experiment, 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 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 dangerous, 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 the lower end is well
above the water, though still immersed in the paraffin. Then allow air
to enter the pipe very slowly 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 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 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.
_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 representation, 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 sounding 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 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 Rayleigh'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 otherwise, 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 portions 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 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 sealing-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.
_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 fascinating 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. 1. 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 pressure
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 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 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 produced, 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 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
neighbouring 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.
_Experiment with Internal Bubbles._
For these experiments, next to a good solution, the pipe is of the
greatest importance. A "churchwarden" is no use. A glass pipe 5/16 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 annoying 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.
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.
[Illustration: Fig. 67. Length of Stem 9 Inches]
[Illustration: Fig. 68. Length of Stem 9 Inches]
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
wire ring should then be hung on to this bubble to slightly pull it out
of shape. For this purpose thin aluminium rings are hardly heavy enough,
and so either a heavier metal should 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. 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 jerk
will almost certainly produce too great a disturbance. A rather rapid
motion, or a slight jerk, is all that is required. It is advisable
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, otherwise 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 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 speaking, enough for a wine-glassful of the soap solution. If
the quantity is increased beyond something like the proportion stated,
the fluorescence becomes less and very soon disappears. The best
quantity can be found in a few minutes by trial.