Since air something better than that of the atmosphere is constantly produced from water by converting it into vapour, and also by removing the pressure of the atmosphere, and these processes do not appear to have any limits; it seems probable, that _water_ united to the principle of _heat_; constitutes atmospherical air; and if so, it must consist of the elements of both dephlogisticated and phlogisticated air; which is a supposition very different from that of the French chemists. LECTURE XXX. _Of Heat._ Heat is an affection of bodies well known by the sensation that it excites. It is produced by friction or compression, as by the striking of flint against steel, and the hammering of iron, by the reflection or refraction of light, and by the combustion of inflammable substances. It has been long disputed, whether the cause of heat be properly a _substance_, or some particular affection of the particles that compose the substance that is heated. But be it a substance, or a principle of any other kind, it is capable of being transferred from one body to another, and the communication of it is attended with the following circumstances. All substances are expanded by heat, but some in a greater degree than others; as metals more than earthy substances, and charcoal more than wood. Also some receive and transmit heat through their substance more readily than others; metals more so than earths, and of the metals, copper more readily than iron. Instruments contrived to ascertain the expansion of substances by heat, are called _pyrometers_, and are of various constructions. As a standard to measure the degrees of heat, mercury is in general preferable to any other substance, on account of its readily receiving, and communicating, heat through its whole mass. _Thermometers_, therefore, or instruments to measure the degrees of heat, are generally constructed of it, though, as it is subject to become solid in a great degree of cold, ardent spirit, which will not freeze at all, is more proper in that particular case. The graduation of thermometers is arbitrary. In that of Fahrenheit, which is chiefly used in England, the freezing point of water is 32 deg., and the boiling point 212 deg. In that of Reaumur, which is chiefly used abroad, the freezing point of water is 0, and the boiling point 80. To measure the degrees of heat above ignition, Mr. Wedgwood has happily contrived to use pieces of clay, which contract in the fire; and he has also been able to find the coincidence of the degrees in mercurial thermometers with those of his own.