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It was about sixty years ago when there appeared the first glimmerings Of the knowledge which has since ripened into that which we now possess, that neither the molecules of any natural object nor the parts Of which those molecules consist are ever at rest; that, on the contrary, swift and orderly movements are ever in progress among them and within them; and that where bodies appear to us to be stationary, it is only be cause this great internal activity is on too small a scale, the parts moving too tiny, and the motions subject to too rapid changes Of direction for senses like ours even when assisted by the micro scope to Obtain any suggestion that all this activity is going forwards. Accordingly, until other means than direct Observation of arriving at the truth were discovered, every one remained under the delusion that the Objects about us on the earth could be brought to rest absolutely freed from every motion except the celestial motion, which is consequent upon their being on a planet which rotates upon an axis, revolves in an orbit round the sun, and accompanies the solar system in its peregrinations through space. In 1811 - nearly a hundred years ago - Avogadro promulgated the important law which bears his name, and which gives expression to the fact that all the more perfect gases, when reduced to the same pressure and temperature, will contain within a given volume the same number of gaseous molecules. The fact was established: but the reason why it is so was not then understood, nor till long afterwards, when in the forties and fifties of the last century some of the activities that go on within gases became gradually known. Until these later dates it was erroneously supposed, even by careful students of nature, that natural objects which to our senses appear at rest - such as stones, coins, books, air which has been left for a long time undisturbed within a room - are in reality devoid of any internal motion. As to gases, one of the illustrations made use of in those days to help students to picture what they were supposed to be like, was that the molecules of a gas may perhaps resemble the stationary bubbles of a froth, which by expanding when warmed, contracting when cooled, and by pressing against one another and against the walls of a containing vessel, behave in these respects very much like a gas. Under this view, Avogadro's Law was expressed by saying that the bubbles, or quasi-bubbles, are all of the same size whatever the gas may be, provided that they are compared with one another when at the same temperature and pressure.

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