Page:Popular Science Monthly Volume 51.djvu/691

Rh liquids and solids, and we shall find that it in no wise falls behind its rivals in activity and prowess. This marvelous little being is a simple particle of air.

Although this particle and its companions wholly escape our vision, they are diffused everywhere around us, and even penetrate our organism to such an extent that without a multitude of them playing a definite part within our body we could not breathe or live for an instant. We can not isolate these particles of air, and could not see them if we did, but we can isolate masses of them by various methods and distinguish them very clearly. Thus let us take a watch glass and a capsule of water, and turning the concavity of the watch glass down, incline it slightly and plunge it into the liquid. Immediately we see a bright line that appears to define the limit of the moistened part of the concave surface of the watch glass. The rest of this face of the glass is kept from being wetted like the whole of the convex surface by the intervention of a mass of particles of air, which, somewhat compressed during the immersion, group themselves into a gaseous globule. Before it was isolated by our maneuver the globule had constituted part of one of the thousands of concentric layers of our atmosphere, each of which weighing upon the one beneath it and communicating to it besides the weight of the layers above it, they all together determine a total even pressure at the level of the sea of fifteen pounds to the square inch. Our globule of air is likewise subject to this pressure, which is transmitted through the water, and added to it is the weight of the liquid that lies above it.

This globule really betrays its presence only by the bright liquid layer around it. When we inquire for the force by which the regular shape of the globule is controlled, we find it, according to the researches of Plateau, in a thin liquid portion surrounding the volume of air, which is not more than one twenty thousandth of a millimetre thick, and which is endowed with a contractile force always impelling it to occupy the least possible space on the body it covers; and by virtue of its curvature it exercises upon the air imprisoned by the liquid a pressure greater in proportion as the globule is smaller. If these dimensions are extremely small, the gaseous globules are always spherical—as, for instance, the bubbles of carbonic acid that rise through a frothy liquor.

Our globule of air imprisoned in the watch glass, acted upon by the pressure of the atmosphere and by that of the liquid above it, and further by the capillary pressure of the bright film encompassing it, possesses, to resist these three pressures, an elastic or repulsive force which is more marked as it is more closely compressed, and by virtue of which the globule occupies a larger