(32.) Since there is a continual change of temperature in all bodies on the surface of the globe, it follows, that there is also a continual change of magnitude. The substances which surround us are constantly swelling and contracting, under the vicissitudes of heat and cold. They grow smaller in winter, and dilate in summer. They swell their bulk on a warm day, and contract it on a cold one. These curious phenomena are not noticed, only because our ordinary means of observation are not sufficiently accurate to appreciate them. Nevertheless, in some familiar instances the effect is very obvious. In warm weather the flesh swells, the vessels appear filled, the hand is plump, and the skin distended. In cold weather, when the body has been exposed to the open air, the flesh appears to contract, the vessels shrink, and the skin shrivels. (33.) The phenomena attending change of temperature are conclusive proofs of the universal porosity of material substances, but they are not the only proofs. Many substances admit of compression by the mere agency of mechanical force. Let a small piece of cork be placed floating on the surface of water in a basin or other vessel, and an empty glass goblet be inverted over the cork, so that its edge just meets the water. A portion of air will then be confined in the goblet, and detached from the remainder of the atmosphere. If the goblet be now pressed downwards, so as to be entirely immersed, it will be observed, that the water will not fill it, being excluded by the _impenetrability_ of the air inclosed in it. This experiment, therefore, is decisive of the fact, that air, one of the most subtle and attenuated substances we know of, possesses the quality of impenetrability. It absolutely excludes any other body from the space which it occupies at any given moment. But although the water does not fill the goblet, yet if the position of the cork which floats upon its surface be noticed, it will be found that the level of the water within has risen above its edge or rim. In fact, the water has partially filled the goblet, and the air has been forced to contract its dimensions. This effect is produced by the pressure of the incumbent water forcing the surface in the goblet against the air, which yields until it is so far compressed that it acquires a force able to withstand this pressure. Thus it appears that air is capable of being reduced in its dimensions by mechanical pressure, independently of the agency of heat. It is _compressible_.