3 o6 THE POPULAR SCIENCE MONTHLY. 



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 un- 

 wonted 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 stag- 

 gering 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 possi- 

 bility of the earth's exploding, we may expect to find fragments 

 of similar explosions, in composition like that of the earth, scat- 

 tered through space. And, in fact, every once in a while as we 

 gaze into the starry heavens we see a flash and exclaim, " A shoot- 

 ing star ! " It is in reality a bit of matter that has come into col- 

 lision 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 frag- 

 ments reach the earth, and are picked up. Such fragments are 

 commonly known as meteorites, and we examine them with ex- 

 treme 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 associ- 

 ated with volcanic activity the igneous rocks as they are called. 

 In general the meteorites are much heavier than the average sur- 

 face 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 com- 

 ponent of meteorites was for years overlooked, but Foote's dis- 



