Thus, for instance, water, though a very common and abundant chemical compound formed by the union of two chemical elements, hydrogen and oxygen, which, at the temperature and pressure of the earth's surface, are gaseous, offers many strange properties to our consideration not shared by other compounds of gaseous elements. For instance, hydrogen, when it combines with gaseous elements other than oxygen, does not form a compound which is liquid at the temperature and pressure of the earth's surface. Its combinations with nitrogen, with chlorine, with fluorine, and even some with the solid element carbon, are under those conditions gaseous. What a special character, therefore, has water! Moreover, water, though a liquid, yet behaves in a most peculiar way when either cooled below ordinary temperatures or heated above them. It becomes solid when cooled, but expands at the same time, so that it is less dense when solid than when liquid--a most unusual proceeding! And when heated it is converted into vapour, but with a loss or "making latent" of heat, which, like its behaviour when solidifying, indicates that water is endowed with a very peculiar structure or mechanism in the putting together of its molecules. We might call these combined peculiarities of water "aquosity," and as we certainly cannot say why water should possess the lot of them, whilst other compounds of either hydrogen or of oxygen, or, in fact, of any other elements, do not possess this combination, we might say that their presence is due to "the aqueous principle," or "aquosity," which enters into water when it is formed, but does not exist in other natural bodies, and, indeed, "sharply separates aqueous from non-aqueous matter."