Few of the minor ingenuities of mankind have amused so many individuals
as the blowing of bubbles with soap-lather from the bowl of a
tobacco-pipe; yet how few who in childhood’s careless hours have thus
amused themselves, have in after-life become acquainted with the
beautiful phenomena of light which the soap-bubble will enable us to
illustrate!
Usually the bubble is formed within the bowl of a tobacco-pipe, and
so inflated by blowing through the stem. It is also produced by
introducing a capillary tube under the surface of soapy water, and so
raising a bubble, which may be inflated to any convenient size. It is
then guarded with a glass cover, to prevent its bursting by currents of
air, evaporation, and other causes.
When the bubble is first blown, its form is elliptical, into which it
is drawn by its gravity being resisted; but the instant it is detached
from the pipe, and allowed to float in air, it becomes a perfect
sphere, since the air within presses equally in all directions. There
is also a strong cohesive attraction in the particles of soap and
water, after having been forcibly distended; and as a sphere or globe
possesses less surface than any other figure of equal capacity, it is
of all forms the best adapted to the closest approximation of the
particles of soap and water, which is another reason why the bubble
is globular. The film of which the bubble consists is inconceivably
thin (not exceeding the two-millionth part of an inch); and by the
evaporation from its surface, the contraction and expansion of the air
within, and the tendency of the soap-lather to gravitate towards the
lower part of the bubble, and consequently to render the upper part
still thinner, it follows that the bubble lasts but a few seconds. If,
however, it were blown in a glass vessel, and the latter immediately
closed, it might remain for some time; Dr. Paris thus preserved a
bubble for a considerable period.
Dr. Hooke, by means of the coloured rings upon the soap-bubble, studied
the curious subject of the colours of thin plates, and its application
to explain the colours of natural bodies. Various phenomena were also
discovered by Newton, who thus did not disdain to make a soap-bubble
the object of his study. The colours which are reflected from the upper
surface of the bubble are caused by the decomposition of the light
which falls upon it; and the range of the phenomena is alike extensive
and beautiful.[15]
Newton (says Sir D. Brewster), having covered the soap-bubble with a
glass shade, saw its colours emerge in regular order, like so many
concentric rings encompassing the top of it. As the bubble grew thinner
by the continual subsidence of the water, the rings dilated slowly,
and overspread the whole of it, descending to the bottom, where they
vanished successively. When the colours had all emerged from the top,
there arose in the centre of the rings a small round black spot,
dilating it to more than half an inch in breadth till the bubble
burst. Upon examining the rings between the object-glasses, Newton
found that when they were only _eight_ or _nine_ in number, more than
_forty_ could be seen by viewing them through a prism; and even when
the plate of air seemed all over uniformly white, multitudes of rings
were disclosed by the prism. By means of these observations Newton was
enabled to form his _Scale of Colours_, of great value in all optical
researches.
Dr. Reade has thus produced a permanent soap-bubble:
Put into a six-ounce phial two ounces of distilled water, and set
the phial in a vessel of water boiling on the fire. The water in
the phial will soon boil, and steam will issue from its mouth,
expelling the whole of the atmospheric air from within. Then throw
in a piece of soap about the size of a small pea, cork the phial,
and at the same instant remove it and the vessel from the fire.
Then press the cork farther into the neck of the phial, and cover
it thickly with sealing-wax; and when the contents are cold, a
perfect vacuum will be formed within the bottle,--much better,
indeed, than can be produced by the best-constructed air-pump.
To form a bubble, hold the bottle horizontally in both hands, and
give it a sudden upward motion, which will throw the liquid into a
wave, whose crest touching the upper interior surface of the phial,
the tenacity of the liquid will cause a film to be retained all
round the phial. Next place the phial on its bottom; when the film
will form a section of the cylinder, being nearly but never quite
horizontal. The film will be now colourless, since it reflects all
the light which falls upon it. By remaining at rest for a minute or
two, minute currents of lather will descend by their gravitating
force down the inclined plane formed by the film, the upper part of
which thus becomes drained to the necessary thinness; and this is
the part to be observed.
Several concentric segments of coloured rings are produced; the
colours, beginning from the top, being as follows:
_1st order_: Black, white, yellow, orange, red.
_2d order_: Purple, blue, white, yellow, red.
_3d order_: Purple, blue, green, yellowish-green, white, red.
_4th order_: Purple, blue, green, white, red.
_5th order_: Greenish-blue, very pale red.
_6th order_: Greenish-blue, pink.
_7th order_: Greenish-blue, pink.
As the segments advance they get broader, while the film becomes
thinner and thinner. The several orders disappear upwards as the film
becomes too thin to reflect their colours, until the first order alone
remains, occupying the whole surface of the film. Of this order the
red disappears first, then the orange, and lastly the yellow. The film
is now divided by a line into two nearly equal portions, one black and
the other white. This remains for some time; at length the film becomes
too thin to hold together, and then vanishes. The colours are not faint
and imperfect, but well defined, glowing with gorgeous hues, or melting
into tints so exquisite as to have no rival through the whole circle
of the arts. We quote these details from Mr. Tomlinson’s excellent
_Student’s Manual of Natural Philosophy_.
We find the following anecdote related of Newton at the above
period. When Sir Isaac changed his residence, and went to live in
St. Martin’s Street, Leicester Square, his next-door neighbour was
a widow lady, who was much puzzled by the little she observed of
the habits of the philosopher. A Fellow of the Royal Society called
upon her one day, when, among her domestic news, she mentioned that
some one had come to reside in the adjoining house who, she felt
certain, was a poor crazy gentleman, “because,” she continued,
“he diverts himself in the oddest way imaginable. Every morning,
when the sun shines so brightly that we are obliged to draw the
window-blinds, he takes his seat on a little stool before a tub
of soapsuds, and occupies himself for hours blowing soap-bubbles
through a common clay-pipe, which bubbles he intently watches
floating about till they burst. He is doubtless,” she added, “now
at his favourite amusement, for it is a fine day; do come and look
at him.” The gentleman smiled, and they went upstairs; when, after
looking through the staircase-window into the adjoining court-yard,
he turned and said: “My dear madam, the person whom you suppose
to be a poor lunatic is no other than the great Sir Isaac Newton
studying the refraction of light upon thin plates; a phenomenon
which is beautifully exhibited on the surface of a common
soap-bubble.”