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.