If any fluid, as water, spirit of wine, or even mercury, be heated in a porous earthen vessel, surrounded by any kind of air, the vapour of the fluid will pass through the vessel _one_ way, while the air passes the _other_; and when the operation ceases, with respect to the _one_, it likewise ceases with respect to the _other_. LECTURE XI. _Of Liquid Substances_; AND FIRST OF _WATER_. Having considered all the substances that are usually found in the form of _air_, I come to those that are generally in a _fluid_ form, beginning with _water_, which is the principal, if not the only cause of fluidity to all the other substances that I shall place in this class. Pure water is a liquid substance, transparent, without colour, taste, or smell; and with different degrees of heat and cold may be made to assume the three forms of a solid, of a fluid, and of air. Below 32 deg. of Fahrenheit it is ice, and above 212 deg. it is vapour; so that in an atmosphere below 32 deg. it never could have been known to be any thing else than a peculiar kind of stone, and above 212 deg. a peculiar species of air. In passing from the state of a solid to that of a liquid, water absorbs a great quantity of the principle, or matter, of _heat_, which remains in it in a _latent_ state; and in passing from a state of fluid to that of vapour, it absorbs much more; and this heat is found when the processes are reversed. It has been observed, that when water becomes vapour, it takes the form of small globules, hollow within, so as to be specifically lighter than air. The degree of heat at which water is converted into vapour depends upon the pressure of the atmosphere; so that in vacuo, or on the top of a high mountain, it boils with little heat; and when compressed, as in Papin's digester, or in the bottom of a deep pit, it requires much heat. In the former case the restoring of the pressure will instantly put a stop to the boiling, and in the latter case the removing of the pressure will instantly convert the heated water into vapour. The ease with which water is converted into vapour by heat, has given a great power to mechanicians, either by employing the natural pressure of the atmosphere, when steam is condensed under a moveable pistern, in an iron cylinder, which was the principle of the old fire-engine, or by employing the elastic power of steam to produce the same effect, which is the principle of Mr. Watt's steam engine. Water was long thought to be incompressible by any external force, but Mr. Canton has shewn that even the pressure of the atmosphere will condense it very sensibly. We do not know any external force equal to that by which water is expanded when it is converted into ice, or into vapour. For though the particles of water approach nearer by cold, yet when it crystallizes, the particles arrange themselves in a particular manner, with interstices between them; so that, on the whole, it takes up more room than before.