needed is the formation of some sort of a crack, to answer to drawing the cork, and fizz, away she goes!
Possibly the thought that we live on top of such effervescent stuff may not be comfortable, for it suggests that the whole earth will some time explode if the volcanoes allowing the gas to escape cease to act as safety valves. There are, indeed, astronomical traces of such catastrophes. Stars have suddenly burst into unwonted radiance, only to fade again almost as quickly; and the belt of asteroids around the sun, occupying as they do a place which naturally would be filled by one large planet, have been supposed to represent some disaster in the process of planet-making. Whether the course of life on this world is ended by the world suddenly exploding, or by a slow refrigeration, or whether the world finally drops into the sun, or is knocked staggering through space by some collision, makes little difference to us, however, so long as the inevitable end that none can foresee must some time come.
Something like this giving off of gas from within the earth is a curious "spitting," as it is called, of molten silver, which when melted absorbs much oxygen gas and gives it off again in cooling. Now, the spectroscope has shown us the kinship in composition of matter throughout the universe, so that stars millions of miles away are composed of the very same elements which make up our own earth and our own bodies. Thus, if we grant the possibility of the earth's exploding, we may expect to find fragments of similar explosions, in composition like that of the earth, scattered through space. And, in fact, every once in a while as we gaze into the starry heavens we see a flash and exclaim, "A shooting star!" It is in reality a bit of matter that has come into collision with our atmosphere at such a tremendous velocity that when so struck even the air resists almost like granite. Indeed, sometimes the shock of collisions dissolves these shooting stars into vapor or dust, but at other times they explode, and the fragments reach the earth, and are picked up. Such fragments are commonly known as meteorites, and we examine them with extreme interest to see if they can throw any light on the average composition of the earth. I think we find that they do, for they are closely allied in composition to some of the series of rocks in the earth's crust that have arisen from beneath and are associated with volcanic activity—the igneous rocks as they are called. In general the meteorites are much heavier than the average surface rocks, and their average weight is much nearer that of the whole earth. The heaviest meteorites are composed mainly of iron—native iron—not quite pure, but associated with some nickel and sulphur and also diamond. This last-named interesting component of meteorites was for years overlooked, but Foote's dis-
