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.