pressure of air is taken off from all about the egg the air in the egg expands, forcing out the contents; but when the air is admitted into the receiver the air in the egg is at once condensed into its former small bulk by the surrounding pressure. [Illustration: Fig. 105.] [Illustration: Fig. 106.] 159. =Hydrostatic Balloon.=--The philosophical toy represented in Fig. 105 illustrates very beautifully the influence of pressure upon the density of the air. The balloon in the jar of water is of glass, with a small orifice at its lower part. Care must be taken in putting water in the balloon to have just enough to make it of a little less specific gravity than water. In that case it will be at the top of the jar, with a very little of its top above the surface of the water. Now tie a piece of India-rubber cloth over the top of the jar, and the apparatus is complete. On pressing upon the India rubber the balloon will go down in the jar, and on taking off the pressure it will rise. The explanation is this: The pressure upon the India rubber is felt through the whole body of the water in the jar, and forces a little more water into the orifice of the balloon, condensing the air that is there. The balloon consequently becomes heavier, and has a greater specific gravity than water, and sinks in it. But when the pressure is taken off, the condensed air in the balloon, by its elasticity, returns to its former bulk, expelling the surplus water just introduced, and the balloon, becoming therefore as light as before, rises. Grotesque figures of glass may be managed in the same way. The Cartesian image, Fig. 106, is an example. This has air in its upper part, _a_, and water up to _c d_. When pressure is made on the India rubber more water is forced into the image through the tail, _b_, and it goes down like the balloon, to rise again when the pressure is taken off. [Illustration: Fig. 107.] [Illustration: Fig. 108.] 160. =Air in Substances.=--I have said that there is air in the pores and interstices of wood, flesh, and a great variety of substances. In all these cases the presence of the air can be made manifest by taking off the pressure of the surrounding air, and thus allowing the air in these substances to expand. If an egg be placed in a jar of water, Fig. 107, under the receiver of an air-pump, on exhaustion being made air-bubbles will constantly rise in the water from the egg. So, too, a glass of porter, Fig. 108, will have its surface covered with foam, the carbonic acid gas in it escaping freely when the pressure of the air upon it is taken off. The same thing may be seen to some extent even in water, for it always contains some air. For the same reason a shriveled apple, with the pressure of the air taken from it, will become plump and fair, but will shrink at once to its shriveled state when the air is admitted into the receiver.