All the substances which we are accustomed to see and handle in our daily lives belong to our world. There are vegetables which grow in the earth, minerals which are dug out of it, and elementary things, such as air and water, which have always made up a part of this planet since man knew it. These are obvious, but there are other things not quite so obvious which also help to form our world. Among these we may class all the elements known to chemists, many of which have difficult names, such as oxygen and hydrogen. These two are the elements which make up water, and oxygen is an important element in air, which has nitrogen in it too. There are numbers and numbers of other elements perfectly familiar to chemists, of which many people never even hear the names. We live in the midst of these things, and we take them for granted and pay little attention to them; but when we begin to learn about other worlds we at once want to know if these substances and elements which enter so largely into our daily lives are to be found elsewhere in the universe or are quite peculiar to our own world. This question might be answered in several ways, but one of the most practical tests would be if we could get hold of something which had not been always on the earth, but had fallen upon it from space. Then, if this body were made up of elements corresponding with those we find here, we might judge that these elements are very generally diffused throughout the bodies in the solar system. It sounds in the highest degree improbable that anything should come hurling through the air and alight on our little planet, which we know is a mere speck in a great ocean of space; but we must not forget that the power of gravity increases the chances greatly, for anything coming within a certain range of the earth, anything small enough, that is, and not travelling at too great a pace, is bound to fall on to it. And, however improbable it seems, it is undoubtedly true that masses of matter do crash down upon the earth from time to time, and these are called meteorites. When we think of the great expanse of the oceans, of the ice round the poles, and of the desert wastes, we know that for every one of such bodies seen to fall many more must have fallen unseen by any human being. Meteors large enough to reach the earth are not very frequent, which is perhaps as well, and as yet there is no record of anyone's having been killed by them. Most of them consist of masses of stone, and a few are of iron, while various substances resembling those that we know here have been found in them. Chemists in analyzing them have also come across certain elements so far unknown upon earth, though of course there is no saying that these may not exist at depths to which man has not penetrated.