=Meteorites.=—There are a multitude of small bodies passing through space in varying directions and with varying velocities and occasionally encountering the earth, to which they add their substance. Some of these meteorites revolve about the sun much as if they were minute planets, but some of them come from such directions and with such velocities as to show that they do not belong to the solar family. Some consist almost wholly of metal, chiefly iron alloyed with a small percent. of nickel (holosiderites); some consist of metal and rock intimately mixed (syssiderites and sporadosiderites); and some consist wholly of rock (asiderites). The rock is usually composed of the heavier basic minerals, though some meteorites consist largely of carbonaceous material. Besides meteorites, there is little doubt that wandering gaseous particles strike the earth, but this is beyond the reach of present demonstration. The amount of substance added to the earth by these meteorites and gases in recent times is relatively slight compared with the whole body of the earth. What contribution may have come to the earth in earlier times from such sources is a matter of hypothesis which will be discussed later. +Geognosy.+ =The constitution of the earth.=—Turning from its external relations to the earth itself, a natural threefold division is presented: (1) the atmosphere, (2) the hydrosphere, and (3) the lithosphere. _I. The Atmosphere._ The atmosphere is an intimate mixture of (1) all those substances that cannot take a liquid or solid state at the temperatures and pressures which prevail at the earth’s surface, together with (2) such transient vapors as the various substances of the earth throw off. The first class form the permanent gases of the atmosphere, and consist of nitrogen about 79 parts, oxygen about 21 parts, carbon dioxide about .03 part, together with small quantities of argon, neon, xenon, krypton, helium, and other rare constituents. The second class are the transient and fluctuating constituents of the atmosphere, chief among which is aqueous vapor, which varies greatly in amount according to temperature, pressure, and other conditions. To this are to be added volcanic emanations and a great variety of volatile organic substances. Theoretically, every substance, however solid, discharges particles which may transiently become constituents of the atmosphere. Practically, only a few of these exist in such quantity as to be appreciable. Dust and other suspended matter are usually regarded as impurities rather than constituents of the atmosphere, but they play a not unimportant part by affecting its temperature and luminosity, and by facilitating the condensation of moisture.