like steel or flint can, if they be heated sufficiently high, not only be transformed into a liquid, but even be driven off into a vapour. Thus we learn that there was a time when our earth was merely a mass of glowing gas.
A great deal of light is thrown upon this subject by looking at other worlds, some of which are to be seen in quite an unfinished state even at the present moment. They are unfinished in the sense that the gaseous material has not yet condensed down sufficiently to form a solid globe. There are thousands of bodies with which astronomers are acquainted which will in one way or another illustrate these phases of our earth's past history. I shall only mention one, which is typical of a remarkable class of similar objects. Fig. 3 represents one of the famous spiral nebulæ, discovered many years ago by the late Earl of Rosse. The object is invisible to the naked eye. It seems like a haze surrounding the stars, which the telescope discloses in considerable numbers, as shown in the picture. When viewed through an instrument oi sufficient power a marvellous spectacle is revealed. There are wisps and patches of glowing cloud-like material which shine not as our clouds do by reflecting to us the sunlight. This celestial cloud is self-luminous; it is in fact composed of vapours so intensely heated that they glow with fervour. As I write, I have Lord Rosse's elaborate drawing of this nebula before me, and on the margin of this stupendous object the nebula fades away so tenderly that it is almost impossible to say where the luminosity terminates. Probably this nebula will in some remote age gradually condense down into more solid substances. It contains, no doubt, enough material to make many globes as big as our earth. Before, however, it
