_The Soap-Bubble.--The Squirt.--The Bellows; An Explanation of
their several parts.--By whom the instrument was invented.--The
sucking and lifting, or common Pump._
“Tom,” said his father, “bring me a saucer with some hot water; a piece
of soap, and a tobacco-pipe. I have promised to teach John the art of
blowing soap-bubbles.”
Tom immediately proceeded to execute his commission, and shortly
rejoined the party on the lawn, bringing with him all the necessary
implements for bubble-blowing. John, under the direction of his
brother, made the lather; and Mr. Seymour, turning towards the elder
children, asked them whether they understood the philosophy of the
operation they had just witnessed; they were, however, unable to return
a satisfactory answer, and their father, therefore, proceeded as
follows:--
“Most liquids, by agitation, exhibit the appearance of froth, in
consequence of the escape of the air in small bubbles, which had been
forced into them by the operation. If, however, the liquid be viscid
and tenacious, like soap and water, the air is, as it were, imprisoned
in the mass, producing the appearance which is commonly called
_lather_.”
Louisa here enquired “Whether the air did not escape with more or less
readiness, according to the degree of resistance it met with in the
liquid?”
“I thank you,” said Mr. Seymour, “for having so kindly assisted me in
the explanation.”
Louisa smiled at this mark of her father’s approbation, and Mr. Seymour
proceeded,--“It is on that very account, that spirit, after it has been
shaken, so soon regains its transparency: for, in consequence of the
superior lightness of that fluid, and the little cohesion which
subsists between its particles, the air makes a rapid escape. In like
manner we may account for the spongy appearance which gives such
superiority to our bread; in that case, the air disengaged during the
fermentation of the dough cannot escape through so viscid a mass; it
therefore remains, and thus produces the eyes or bubbles, which you may
always observe in every well-baked loaf.”
“See, papa,” exclaimed Tom, “the bubbles which John has blown in the
lather, are not round, but angular figures--they appear to be like the
hexagons which we used to cut out for our _papyro-plastics_.”
“They are certainly hexagonal,” replied Mr. Seymour; “and the form
arises from the pressure of the bubbles upon each other. The same
appearance is to be seen in the pith of vegetables, when examined by
the microscope, and is the result of the general reaction of the solid
parts upon each other: but let us proceed to blow some bubbles. Plunge
the bowl of the tobacco-pipe into the lather.”
Tom obeyed his father’s directions, and blowing through the stem
produced a bubble.
“See! see!” cried Louisa, “what a beautiful bubble! but there is a
quantity of soap hanging to its under part.”
“I will take it off with my finger,” said Mr. Seymour.
“There it goes!” exclaimed Tom.
“What beautiful colours it displays! as bright and gaudy as those of
the rainbow!” observed his sister.
“It has burst!” cried Louisa.
“Ah! my dear children,” murmured the vicar, with an air of pensive
gravity, “‘_Tenues secessit in auras_,’ as the poet has it. Even thus
is it with all the full-blown bubbles of our fancy, raised by the
breath of hope; the moment they appear most vivid and promising to our
imagination, they vanish ‘into air, into thin air,’ like the gaudy and
unsubstantial soap-bubble you have just witnessed: but proceed to blow
another.”
“There is one!” exclaimed Louisa;--“see, it is of an oblong shape, like
an egg!--there it goes!--but I declare it is now perfectly
round!!--what can be the reason of its changing its figure?”
“I am glad you have asked that question, because my answer will serve
to illustrate an important property of air, and which, indeed, is
common to all fluids. While the upper part of the bubble was attached
to the bowl of the pipe, its gravity being resisted, drew it into an
elliptical form; but the instant it was detached, the contained air
pressed equally in all directions, and the bubble, in consequence,
became a perfect sphere.”[38]
“I do not exactly understand what you mean ‘by pressing equally in all
directions.’”
“The expression is surely sufficiently intelligible. Did you not learn
in our conversation of yesterday, that air has weight, and exerts a
pressure as much upwards as downwards and laterally? Were this not the
case, how could the air in the interior of our bodies counteract the
pressure of the atmosphere? The form of the bubble proves the same fact
in a different way; for, had the air in its cavity pressed more in any
one direction than in another, the bubble could not have been round,
or, to speak more correctly, a sphere.”
“What are you musing about?” cried the vicar, who had observed the
attention of the boy riveted upon the bowl of the tobacco-pipe: “I am
sure, from your countenance, that some circumstance is puzzling you.”
“You are right, my dear sir; I was just then thinking how it can
possibly happen, that the bubble should not have a hole in its upper
part; for, while I am blowing it up, there must, of course, be a
communication between my mouth and its interior, or else how could the
air pass into it?”
“True,” said his father; “but the act of throwing it off from the bowl
of the pipe will unite this breach; for there exists a strong cohesive
attraction between the attenuated particles of the lather; you will,
therefore, perceive that, on this account, the bubble will be more
readily and securely separated by a lateral than a perpendicular motion
of the pipe.”
“I wish,” said Tom, “that I could discover some method of preventing
their bursting so soon, for there is scarcely time to examine them
before they vanish. What can be the cause of their short duration?”
“Consider, my dear boy, the frailty of their structure, and I think
that the precarious tenure of their existence will cease to astonish
you; indeed, the wonder is, that they should endure so long. The film
of which they consist is inconceivably thin,[39] so that the slightest
impulse will be apt to rupture them; besides which, there must be a
considerable evaporation going on from their surface, while the
contraction of the contained air, from change of temperature, must also
tend to limit their duration. You must likewise remember that the
soap-lather will have a tendency to gravitate towards the depending
part of the bubble, and, consequently, by quitting the upper portion,
to render it of still greater tenuity. This last effect might, perhaps,
be obviated, in some measure, by giving a rotatory motion to the bubble
around its axis; but this, again, would accelerate the evaporation:
which, after all, is the principal cause of the shortness of its
duration; so that, unless this latter effect could be remedied, I
despair of suggesting any expedient by which the frail existence of our
airy structure could be protracted. You must, therefore, seek, from a
succession of bubbles, the prolongation of an amusement which no single
one can afford you.”
“And could not the evaporation be prevented?” asked Tom.
“If the bubble were blown in a glass vessel, and the latter immediately
closed after the operation, it would remain for some time; I remember
having once preserved a bubble in this manner for a very considerable
period.”
Tom, however, did not appear to relish this scheme; as, he said, the
great sport arose from watching the movements of the floating bubble;
the boy, accordingly, determined to pursue the amusement in the usual
manner. His father, however, observed, that by mixing a solution of
isinglass with the soap lather, larger, as well as more lasting bubbles
might be blown; and Tom accordingly determined to make the experiment.
During this dialogue, little John had succeeded, for the first time, in
launching the airy bauble. Imagination always tinges the objects of our
first efforts with brilliant tints: no wonder, therefore, that John,
with a shout of ecstasy, should have pronounced it to have been the
most beautiful bubble he had ever seen: in truth, the sun was shining
brightly, and the colours thus produced very justly excited the
admiration of all present.
“I cannot understand the cause of these beautiful colours,” said Louisa.
Mr. Seymour expressed a fear that, in their present state of knowledge,
they would be scarcely able to understand the explanation he should
afford them. “But,” said he, “I believe you know that a ray of light is
divisible into seven colours, and that when it passes through certain
media, or is reflected from certain surfaces, this division is
effected, and the various colours produced(32). The film of the soap
bubble is one amongst the latter bodies; but I must refer you, for
farther information upon this subject, to Mrs. Marcet’s beautiful
account of ‘Refraction and Colours.’”
“Now, Tom,” said his father, “fetch your squirt, for we have not yet
finished our enquiry into the effects of the air’s pressure.”
The squirt was produced; but it was out of repair: for, on attempting
to fill it with water, the instrument entirely failed in the
performance of its office.
“I see the defect,” said Mr. Seymour, “which a little string will
easily remedy.”
A piece of string was instantly produced from that universal depot, the
breeches pocket of a school-boy. Mr. Seymour said he should bind a
portion of it around the end of the piston.
“What do you mean by the _piston_?” enquired Tom.
“The rod which moves up and down in the cylinder, or tube; and, unless
its end fit so exactly as to prevent the admission of air, it is clear
that the squirt cannot draw any water. It was for the purpose of making
this part fit tightly that I wanted the string, and you will now
perceive that the instrument is ready for use:--fetch me a vessel of
water.”
Tom soon produced the water, and, on placing it on the ground,
requested that he might be allowed to fill the squirt. This he
accordingly effected without difficulty, and, on pressing down the
handle, he projected a stream of water to a considerable distance.
“I perceive,” said Tom, “that the stream describes a curve, like my
ball.”
“To be sure; it is under the joint influence of the same forces, viz.
that of projection and of gravity. But explain the operation of the
squirt.”
“As soon as I raised the piston, an empty space was left in the lower
part of the cylinder, which I suppose would have remained as a
_vacuum_, had not the water rushed into it.”
“And why did the water rush into it?”
Tom hesitated.
“Was it not, think you, owing to the pressure of the atmosphere upon
the surface of the water? When you raised the piston, the air above it
was also raised, and ultimately driven out by the force of the
ascending piston; and since the air could not find any entrance from
below as long as the point was under the water, the interior of the
squirt would necessarily have remained quite empty, or have been a
vacuum, had it not been for the weight of the atmosphere, which, not
having any counteracting pressure, drove the water into the tube, and
thus filled it; and which, by pressing down the piston, you again
expelled with considerable force.”
“Your explanation,” cried Louisa, “is so clear and intelligible, that I
feel quite confident I could now explain any machine that owes its
action to the exhaustion of the air, and the pressure of the
atmosphere.”
“If that be your belief,” said Mr. Seymour, “I will not lose a moment
in putting your knowledge to the test.--Tom, do you run into the house,
and fetch hither the kitchen bellows.”
The bellows were produced, and Louisa having been desired by her father
to explain the manner in which they received and expelled the air,
proceeded as follows: “Upon raising the upper from the under board, the
interior space of the bellows is necessarily increased, and immediately
supplied with an additional quantity of air, which is driven into it by
the pressure of the atmosphere; when, by pressing down the upper board,
it is again expelled through the iron tube or nosle.”
“To be sure,” said Tom, “in the same manner that the water was expelled
from my squirt, when I pushed down the handle.”
“So far you are quite correct,” said Mr. Seymour; “but you have not yet
told us the use of the hole in the under-board, and which is covered,
as you perceive, with a movable flap of leather: it is termed a valve,
or ‘_wind-clap_.’”
“That,” replied Tom, “is for the purpose of admitting the air, when we
raise up the board.”
“Exactly so; and also to prevent the air from passing out again, when
you press it down. I wish to direct your attention particularly to this
contrivance, because, simple as it may appear, its action will teach
you the general nature of a valve. Without it, the operation of filling
the bellows with air would have been so tedious as to have destroyed
the utility of the instrument; for the air could, in that case, have
only found admission through the nosle, and that, again, would have
been attended with the additional disadvantage of drawing smoke and
other matter into its cavity; when, however, you raise up the board,
the air, by its external pressure, opens the wind-clap inwards, and
thus finds an easy entrance for itself; and when you press the board
downwards, the air, thus condensed, completely shuts the valve, and its
return through that avenue being prevented, it rushes out through the
tube.”
The children were much pleased with the simplicity of this invention,
and Tom enquired of the vicar who first thought of it.
“We are informed by Strabo,” replied Mr. Twaddleton, “that Anacharsis,
the Scythian philosopher, who lived in the time of Solon, about six
hundred years before Christ, invented the bellows, as well as the
anchor, and potter’s wheel; but,” he added, “there is some reason to
doubt the truth of this statement. The bellows, however, were certainly
known to the Greeks; and the great poet Virgil alludes to them in his
fourth Georgic:[40]
----‘Alii taurinis follibus auras
Accipiunt redduntque.’”
Mr. Seymour now proposed that they should proceed to consider the
structure and operation of the pump.
“I suppose,” said Louisa, “that the pump raises water in the same
manner as the squirt.”
“Exactly upon the same principle,” replied her father; “but the
machinery is a little more complicated, since its object is not to
force the water out of the pump at the same end of the pipe at which we
draw it in. We will, however, proceed to the stableyard, and examine
the pump; and do you, Tom, provide a piece of chalk, in order that I
may make a sketch of some of its principal parts.”
The party immediately proceeded; and, as they walked along, Mr. Seymour
desired the children to remember that the weight of the atmosphere was
estimated as being equal to that of fifteen pounds upon every square
inch of surface; and that the moment the water arrived at such a height
as to balance that pressure, it could ascend no higher: he added, that
the altitude at which such a balance took place was about 32 or 33 feet
above the surface.
“If that be the case,” said Louisa, “the pump, of course, can never
raise water from any well of greater depth than that which you state.”
“Not without some additional contrivance, which I shall afterwards
explain to you,” replied Mr. Seymour.
The party had, by this time, arrived at the pump; its door was opened,
and as much of the apparatus exhibited as could be conveniently
exposed. Mr. Seymour then chalked the annexed sketch upon the stable
door.
[Illustration: Diagram of a water-pump’s mechanism.]
“Is that a pump?” asked Tom: “I should certainly never have guessed
what you intended to represent.”
“It is not a perspective drawing, my dear, but a representation of the
different parts as they would appear, were it possible to cut the pump
in halves, from top to bottom, without disturbing any of its
arrangements. A drawing of this kind, which is frequently used for the
sake of explanation, is termed a _section_.”
Mr. Seymour here took an apple from his pocket, and having cut it in
two, observed that the surfaces thus exposed presented _sections_ of
the fruit. This illustration was understood by all present, and Mr.
Seymour continued, “I have here, then, a section of the common
household pump. A B is the cylinder or barrel; P the air-tight piston
which moves or works within it, by means of the rod; Q is the
‘suction,’ or ‘feeding pipe,’ descending into a well, or any other
reservoir; S the valve, or little door, at the bottom of the barrel,
covering the top of the feeding pipe; and there is a similar valve in
the piston, both of which, opening upwards, admit the water to rise
through them, but prevent its returning. As this part of the apparatus
is no less ingenious than it is important, I will sketch the valve, or
_clack_, as it is termed by the engineer, on a larger scale.”
Their father then chalked the annexed figure; from which its
construction was rendered perfectly intelligible to the children.
[Illustration: Open valve.] Mr. Seymour proceeded: “When the pump is in
a state of inaction, the two valves are closed by their own weight;
but, on drawing up the piston P, from the bottom to the top of the
barrel, the column of air, which rested upon it, is raised, and a
vacuum is produced between the piston and the lower valve, S; the air
beneath this valve, which is immediately over the surface of the water,
consequently expands, and forces its way through it; the water then
ascends into the pump. A few strokes of the handle totally exclude the
air from the body of the pump, and fill it with water: which, having
passed through both valves, runs out at the spout.”
“I understand how water may be thus raised to the elevation of 32 feet,
but I have yet to learn the manner in which it can be raised above that
distance,” said Louisa.
“It is undoubtedly true that, if the distance from the surface of the
water to the valve in the piston exceed 32 feet, water can never be
forced into the barrel; but you will readily perceive that, when once
the water has passed the piston valve, it is no longer the pressure of
the air which causes it to ascend; after that period, it is raised by
lifting it up, as you would raise it in a bucket, of which the piston
formed the bottom; and water, having been so raised, cannot fall back
again, in consequence of the valve, which is kept closed by its
pressure. All, therefore, that is necessary, is to keep the working
barrel within the limits of atmospheric pressure; we have then only to
fix a continued straight pipe to the top of the barrel, and to lengthen
the piston rod in the same proportion, and the water will continue to
rise at each successive stroke of the pump, until at length it will
flow over the top of the pipe, or through a spout inserted in any part
of its side. The common pump, therefore, is properly called the
_sucking_ and _lifting pump_.”
The party expressed themselves fully satisfied; and Tom enquired who
invented the machine.
“It is an instrument of great antiquity,” replied his father: “its
invention is generally ascribed to Ctesebes of Alexandria, who lived
about 120 years before Christ; but the principle of its action was not
understood for ages after its invention. The ancients entertained a
belief that ‘Nature _abhorred_ a vacuum;’ and they imagined that, when
the piston ascended, the water immediately rushed forward to prevent
the occurrence of this much dreaded vacuum. In the seventeenth century
a pump was constructed at Florence, by which it was attempted to raise
water from a well to a very considerable altitude, but it was found
that no exertion of this machine could be made to raise it above 32
feet from its level. This unexpected embarrassment greatly puzzled the
engineer, until Galileo suggested that the pressure on the water below
must cause its ascent into the pump, and that, according to this
theory, when it had risen 32 feet, its pressure became equivalent to
that of the atmosphere, and could, therefore, not rise any higher; and
as they did not, at that time, understand the construction of the
piston valve, the design was abandoned. It is now time to conclude your
lesson. To-morrow I hope we shall be able to enter upon the subject of
THE KITE.”
-----
Footnote 38:
A scientific friend observed to the author, that, as the globe
possesses less surface than any other figure of equal capacity, it is
of all forms that which is best calculated to allow the closest
approximation of the particles of soap and water; and as there must
exist amongst such particles a strong cohesive tendency, after having
been forcibly stretched out, as it were, by the air blown into the
bubble, it follows that, did no other cause operate, the bubble would
assume the spherical form.
Footnote 39:
Not exceeding the two millionth part of an inch.
Footnote 40:
Line 171.
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[Illustration: Two boys causing havoc by upsetting a washerwoman.]