Utterly different in all respects are the nebulæ. They are not masses of watery vapor. It may no doubt possibly be that water in some form is there, but it is not water which we see. We are looking at some gaseous material of a bluish hue. The light with which it glows is no reflected sunlight. The nebula is indeed indebted to no foreign source for that weird—I had almost said ghostlike—radiance which it gives forth. The light comes from the nebula itself. But how, it may well be asked, should a purely gaseous substance be able to radiate forth light? It is easy for us to comprehend how stars or suns or comparatively solid bodies can, in virtue of their tremendous temperature, glow with heat like red-hot or white-hot iron. It is true that flame is gas in an incandescent state, but in flame a vehement chemical union of oxygen with some other substance is in progress, and this is the source of the heat and the light that flame gives forth. We can not regard the Great Nebula in Orion as originating in anything resembling flame. We can, however, in our physical laboratories arrange an experiment which seems to throw some light on the composition of the nebula. Into a glass tube a small quantity of hydrogen gas is admitted, the air having been previously extracted. Then, by means of two wires, one at each end of the tube, an electric current is transmitted through the gas. Here there is no combustion; the gas is merely the vehicle by which the electricity flows from one pole to the other. In doing so the gas instantly begins to glow with an intense bluish light, and a very beautiful effect is produced, which can be renewed or terminated at will by simply making or breaking the electric current. It would seem as if the gas we see in the nebula were in a condition somewhat analogous to the gas in the tube. I do not mean that the passage of electricity through the nebula is the source of its luminosity. There is, indeed, no ground for such a supposition. It is the property of electricity when passing through a conductor to warm that conductor; thus we know that if a powerful current be transmitted through a wire of the most infusible of all metals, platinum, the wire will not only get warm, but it may become red hot, white hot, and even melt under the influence of the heat which is generated. In those beautiful incandescent electric lamps which are now happily coming into extensive use a current of electricity flows through a filament of carbon, and kindles that exquisite incandescence which is maintained while the current flows. It would appear that so long as the electricity is flowing through the glass tube its action on the gas is to impart a very high temperature. It is in consequence of this temperature that the gas glows. Now we can offer a reasonable account of the luminosity of the Great Nebula in Orion. The particles of gaseous or vaporous material of which it is formed are of an extremely high temperature, sufficient to enable