Blowing soap bubbles may strike you as a very childish amusement, but
from them there is much to be learned of scientific interest.
The soap bubble, as ordinarily blown, by means of a bubble pipe
contains within it carbon dioxide gas, given off by the lungs. The
coat of the soap bubble, composed of a mixture of soap and water,
is elastic, and capable of being stretched or pulled in different
directions without being broken. In order to give greater strength and
elasticity to this film, a mixture of soap, water and glycerine may be
employed. Common yellow soap is better than most of the fancy soaps.
Castile soap will furnish an excellent soap bubble mixture.
If the soap bubble be examined, it will be found that various beautiful
colors are reflected upon its surface. These colors are usually very
delicate, and consist largely of pinks, blues and greens. After a few
seconds, a darkish spot begins to appear at one point on the surface
of the bubble. This spot becomes darker until it is almost black, and
almost immediately after this the bubble bursts.
This black spot is in reality the thinnest portion of the soap bubble,
and it is at this point that the film gives way first. The soap bubble
skin has at this point become so thin that it will no longer reflect
light-rays.
Soap bubbles have even furnished us a means of determining the size of
the molecules of matter. Knowing the amount of soap and water employed,
in order to produce a definite bubble, the size of the bubble, when
blown, is measured, and in this way the number of molecules of
soap and water in any given square centimetre of its surface can be
calculated. The thickness of the film may be determined by the coloring
of the bubble. Knowing the area and thickness of the soap bubble film,
the number of molecules can be calculated, and in this way it has been
ascertained that approximately a given number of molecules of matter
are contained within that space.
This elastic coat, which we see in soap bubbles, may also be noted in
other mixtures, such as alcohol and water. The strength of this skin
may be seen on the side of a wine glass, in which there is some fairly
strong wine, such as port. The liquid is observed to climb up the sides
of the glass, and then to gather into drops and 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, becomes 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 run back
again into the glass.
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
its color in the cup, when it moveth itself aright.”
Much more might be said on this interesting topic, but the above will
at least serve to show us that we can learn facts of great interest
even from the lowly soap bubble.