Children sometimes ask, and men and women need not be ashamed to ask,
why is froth always white or nearly white, whatever may be the colour
of the liquid underneath it? To answer the question, we shall have to
determine what froth really is in itself, and how it is formed.
Take a filled ale-jug, or the well-known 'pewter' of a tavern or
public-house, and pour out gently into a glass: scarcely any froth is
produced. Pour out the self-same liquor from the self-same vessel in a
stream several inches high, and you produce a foaming 'head,' which to
the eye seems to be a substance quite different from ale or beer. Open
a bottle of lemonade or ginger-beer, of soda-water or seltzer-water,
and pour out the contents into a glass; the formation of froth is so
rapid and abundant that the glass appears full when it really contains
but a small quantity of liquid. Open a bottle of Bass's ale or of
Guinness's stout; the froth is still more opaque and pronounced. Look
at a cup of tea or coffee soon after the sugar has been added; there
will often be seen a small covering of froth on the surface, which
froth, if not actually white, is much more so than the liquid beneath.
Watch the movements of the paddle-wheels of a steamer; the water
thrown back from them is covered with foam of dazzling whiteness, the
intensity of the white increasing with the rapidity of the wheel's
motion.
In all these cases, and in multitudes of like kind, the froth seems
to result from agitation; a quiescent liquid seldom presents symptoms
of this nature. But agitation alone would not do it. Supposing it
were possible for human beings to live, or for pints of ale to exist,
without _air_, there would be no such thing as froth. In pouring out
ale or other liquid, the falling stream becomes mingled with a portion
of the atmospheric air which surrounds us; and it also buries, as it
were, the air contained in the apparently empty glass: the result
is, a mixture of ale and air, instead of ale only. Under ordinary
circumstances, liquids contain as much air as they are capable of
absorbing; the additional quantity is expelled. But how? It cannot rise
in a body to the surface, but divides into minute spherical portions or
air-bubbles, which ascend to the top of the liquid on account of their
levity or comparative lightness.
This, however, is not all; if it were, the bubbles would burst directly
they come to the surface, and froth would not have an opportunity of
forming. Cohesion comes into action, the cohesion between the particles
of every liquid. The bubbles of air, as they rise from the body of
the ale, beer, or other liquid, are able to penetrate between the
particles; but when they arrive at the surface they encounter a film of
liquid cohesive enough to restrain for a time their final escape into
the atmosphere. Froth consists of bubbles, each a tiny globular portion
of air, bounded on the upper surface by an exceedingly thin film of
liquid; the bubbles retain this position and condition until the
struggle ends as a victory for one of the belligerents. The ascensive
power of the air within the bubble overcomes the cohesive power of the
liquid film or covering, and 'the bubble bursts'--our beverage loses
its frothy 'head.' The length of time that elapses before the air
escapes by the bursting of the bubble depends chiefly on the viscidity
of the liquid. If we pour ale into one vessel and water into another,
from equal heights, we shall find that the former will present a frothy
surface, the latter a more instantaneous sparkling appearance; the
power of retention being greater in the former instance than in the
latter. In other words, ale having a greater specific gravity than
water, and also greater viscidity, the bubble formed has a thicker
coating, and requires a longer time for its disrupture. Taking the
round of all the 'frothy headings,' we find the same rule prevail. In
the spray from the paddle of a steamboat, in the froth on the surface
of beverages, in the sparkles on a cup of tea, the air is in the first
place entangled among the particles of liquid, and thus forms bubbles
in the struggle to escape. Although carbonic acid may, in regard to
physical properties, be ranked as a kind of air, the formation of froth
in effervescing liquids is slightly different.
But why is froth _white_? Porter, ale, tea, coffee, champagne, water,
differ very considerably in colour; yet in all of these, when froth is
produced it is white. This appears to be due to the reflection of the
light from the outer surface of the several bubbles. When the surface
is thus broken up, we have a cluster of little spheres, each of which
presents a reflection to the eye from some part of its surface; and
as there may be tens of thousands of these in a very small space, the
effects become united, and are recognised as a whiteness. It is simply
an aggregation of small effects to produce one more conspicuous. If the
bubbles are large, then fewer of them can be contained in a given area;
consequently the number of convex reflecting surfaces is smaller, and
the united effect less brilliant--in other words, less white.
But it may still be asked by some of us, how is it that the froth of a
_reddish_ liquid, such as beer, is white? The phenomena of reflected
light must again be appealed to for the means of solving this problem.
The colour of a liquid (not its froth) is determined by the transmitted
light, not the reflected. If liquor be contained in a transparent
glass vessel held between the eye and the light, and we look _through_
it, the eye receives the light transmitted by the liquid, and deems
the colour of that light to be the colour of the liquid itself; but
if we pour the liquor into an open vessel, and look obliquely at the
surface, we shall find that the colour does not deviate much from
whiteness, whatever the transmitted colour may be. The liquid, whatever
be its body colour, is when agitated broken up into detached portions
at its surface by the formation of bubbles, and each bubble reflects
to the eye a portion of the light which falls upon it. Consequently,
if this reflected light is nearly white in all cases, the resulting
assemblage of bubbles, generally known as froth, must always appear
white or nearly white. We can easily understand the greater yellowness
of the froth on strong Dublin stout than that on pale Burton ale; the
more tenacious liquor forms a thicker and consequently less clearly
reflecting bubble than that on the more limpid.