Page:Popular Science Monthly Volume 38.djvu/86

76 it has been supposed, by tidal action, as they approach the sun; because such an action must make a considerable difference in the movements of the particles of the swarm nearer the sun, as compared to those farther away from it; we know, in any case, by their increased light, that the temperature of comets does increase considerably as the sun is approached. It has been shown that many of the phenomena presented by comets, which are acknowledged to be clouds of meteoritic particles in the solar system, are identical with those presented by nebulæ and stars in space; hence the hypothesis now under consideration, which affirms the nebulæ to be also clouds of meteoritic dust, is greatly strengthened. Indeed, if the facts had not been found to be as I have stated them, the hypothesis would have been worth nothing.

I should here add that the recent work has shown how right Schiaparelli was, when, in 1866, he stated that comets were nebulous masses drawn into the solar system.

The top of what we agreed to call the temperature curve may now be considered. We have dealt with the ascending arm of it, and referred to the groups I, II, and III. In these groups there was evidence to show that, under normal conditions, we were dealing with orders of celestial bodies in which the temperature was gradually increasing, in consequence of the continual nearing of the constituent meteorites in the swarm due to collisions and gravitation.

It may be convenient that I should very briefly give, even at the risk of being charged with repetition, a normal case carrying us up to the top of the curve. For that purpose we may content ourselves by considering those globular and elliptic nebulæ first recorded by Sir William Herschel in the last century. In these there is evidence of different stages of condensation; in one series first of all something which is hardly visible is noted, and the end of that series consists of a dim, diffused, globular mass. In another we pass from the minimum gradually into another form of condensation, in which the luminosity increases toward the center. In still another series the condensation toward the center goes as it were by jumps, so that finally what appears to be a nebulous star with a surrounding of very nearly equal density is seen. Passing from these forms we come to elliptic nebulæ, which doubtless indicate a further condensation of those forms which, in the first instance, are globular. We have already become familiar with a representative of these elliptic nebulæ in that of Andromeda, as it has been revealed to us by the magnificent photograph taken by Mr. Roberts. In connection with such an elliptic figure we often get clear indications of spirals.

A further condensation then will no doubt land us among stars having a peculiar and special spectrum; indeed, though they