The regular and constant constituents of the atmosphere are the elements oxygen and nitrogen, which, at the temperature and pressure existing on the surface of our globe, are permanently aeriform or gaseous. Beside these gases, the air always contains a quantity of the vapor of water in a perfectly aeriform and transparent condition. This vapor is not, however, permanently gaseous. At all temperatures below 212 degrees it tends to the liquid state; and its elastic force, which preserves its particles in the separated state of vapor, increases or diminishes at a more rapid rate than the increase or diminution of temperature. Hence the quantity of vapor that can be suspended in clear air depends on the temperature of the air itself. As the temperature of the air rises, its power of sustaining vapor increases more rapidly than its temperature; and as the temperature of the air falls, the elastic force of its contained vapor diminishes in a greater ratio, until it can exist as an invisible vapor no longer, but becomes condensed into minute bubbles or globules, forming cloud, mist, or rain. Two other circumstances operate along with these properties of air and vapor. The heat radiated from the earth's surface causes the lower strata of air to be, in ordinary circumstances, warmer than the higher; and, on the other hand, warm air, being lighter than that which is colder, the warm layer of air at the surface continually tends to rise through and above the colder currents immediately over it. Let us consider the operation of the causes thus roughly sketched in a column of calm air. The lower portion becomes warmed, and if in contact with water takes up a quantity of its vapor proportioned to the temperature, or in ordinary circumstances somewhat less than this proportion. It then tends to ascend, and as it rises and becomes mixed with colder air it gradually loses its power of sustaining moisture, and at a height proportioned to the diminution of temperature and the quantity of vapor originally contained in the air, it begins to part with water, which becomes condensed in the form of mist or cloud; and the surface at which this precipitation takes place is often still more distinctly marked when two masses or layers of air at different temperatures become intermixed; in which case, on the principle already stated, the mean temperature produced is unable to sustain the vapor proper to the two extremes, and moisture is precipitated. It thus happens that layers of cloud accumulate in the atmosphere, while between them and the surface there is a stratum of clear air. Fogs and mists are in the present state of nature exceptional appearances, depending generally on local causes, and showing what the world might be but for that balancing of temperature and the elastic force of vapor which constitutes the atmospheric firmament.[68]