THE FORCES IN AN AIR BUBBLE. 675 



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 pene- 

 trate 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 turn- 

 ing 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 gase- 

 ous 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, accord- 

 ing to the researches of Plateau, in a thin liquid portion sur- 

 rounding 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 ex- 

 ercises upon the air imprisoned by the liquid a pressure greater in 

 proportion as the globule is smaller. If these dimensions are ex- 

 tremely 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 encom- 

 passing it, possesses, to resist these three pressures, an elastic or 

 repulsive force which is more marked as it is more closely com- 

 pressed, and by virtue of which the globule occupies a larger 



