Page:Popular Science Monthly Volume 16.djvu/772

738 crosses the orbits of certain meteoric swarms, we have showers of shooting stars, fortunately so small that their bombardment is unnoticed.

Scarcely anything is known, or plausibly guessed, concerning the condition and properties of nebulous matter. If, for example, the



spectrum of a nebula indicates hydrogen, we may be pretty sure it is not in the state of the gas as it is known in our laboratories. The recent discoveries of Crookes concerning the properties of matter a million times more attenuated than common air lead to the hope that fresh light may be thrown upon many astronomical questions; but in the mean time it is impossible to form more than a vague idea of the condition of any star or planet that does not in its main features resemble our earth; and this can be said only of Mars, on whose globe we can discover what is probably land and what is water, and see white masses, which it is reasonable to believe are snow, form and melt away as the planet's winter and summer affect them in turns.

Our earth has long been in a state of slow, as distinguished from that of rapid, change. The geologist finds the oldest rocks he can discover affording indications that they were formed when the circumstances of the globe were sufficiently like what they are now for fair comparison. The earth's surface may have been warmer, its atmosphere more moist, and it may have contained more carbonic acid than we now find; storms may have been more frequent and more violent, but the assemblage of differences between what now is and what was