'The short and simple annals of the poor.'
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.