The meteorites or foreign bodies which bombard the earth may be included in three classes--meteoric irons or aërosiderites, meteoric iron stones or aërosiderolites, and meteoric stones, aërolites--all containing elements, about twenty-five in number, which have been found upon the earth. The most conspicuous and important are silicon, iron, nickel, magnesium, sulphur, carbon, and phosphorus, while the others are aluminum, antimony, arsenic, calcium, chlorine, chromium, cobalt, copper, hydrogen, lithium, manganese, oxygen, potassium, sodium, tin, and titanium. Hydrogen and the diamond have also been observed. A number of interesting chemical compounds are found in meteorites not known on the earth, and a study of their character shows that the conditions under which the meteors were formed were entirely different from those which saw the beginning of things terrestrial. In brief, where meteors were born there was an absence of air and water. On the other hand, there was at some stage in the history of meteorites an abundance of hydrogen. The meteoric irons are made up principally of iron with an alloy of nickel, and show a rich crystalline structure, the various angles producing a variety of forms known as _Widmanstatten_ figures which a few years ago formed the basis of a singular sensation. The figures were supposed to be fossil shells and various animals of a diminutive size which once populated the wrecked world of which the meteor was assumed to be a part. These meteoric animals from space were named and classified by several observers, who were finally forced to acknowledge that their creations were the fanciful markings of crystallization. Another class of meteorites (meteoric iron stones) may be described as spongy masses of nickeliferous iron in whose pores are found grains of chryosite and other silicates. A type of these bodies is the meteor of Pallas, which was discovered by him in 1772. The third class of meteoric stones are those in which the stony or silicous predominates. As a rule they contain scattered metallic grains, but certain ones, as the aërolite which fell at Gara, France, in 1806, contain metallic constituents.