That there are some of the nebulæ which appear to consist of solid matter interspersed in a gaseous mass is shown by the researches of Mr. Lockyer[10] and others. In fact, the theory is held by Professor Tait[11] that nebulæ consist of clouds of stones—or meteor-swarms, as Mr. Lockyer would term them—in an atmosphere of hydrogen, each stone of which, moving about and coming into collision with some other, is thereby generating heat which renders the circumambient gas incandescent. In reference to this theory of Professor Tait, Mr. Lockyer says that the phenomena of the spectroscope can be quite well explained “on the assumption of a cloud of stones, providing always that you could at the same time show reasonable cause why these clouds of stones were ‘banging about’ in an atmosphere of hydrogen.”[12] The theory, however, does not appear to afford any rational explanation of this banging about of the stones to and fro in all directions; for, according to it, the only force available is gravitation, and this can produce merely a motion of the materials towards the centre of the mass. Under these conditions very little impinging of the stones against each other would take place. But, according to the theory here adopted, we have an agency incalculably more effective than gravity, one which accounts not merely for the impact of the stones, but for their very existence as such, inasmuch as it explains both what they are and whence they came. [Footnote 10: _Proceedings of Royal Society_, vol. xliii. p. 117.] [Footnote 11: _Good Words_ for 1875, p. 861.] [Footnote 12: _Manchester Science Lectures._] Mr. Lockyer has recently fully adopted Professor Tait’s suggestion as to the nature and origin of nebulæ, and has endeavoured to give it further development. He considers the nebulæ to be composed of sparse meteorites, the collisions of which give the nebulæ their temperature and luminosity. He divides the nebulæ into three groups, “according as the formative action seems working towards a centre; round a centre in a plane, or nearly so; or in one direction only.” As a result we have globular, spheroidal, and cometic nebulæ. _Globular nebulæ_ he accounts for in the following manner. “If we,” he says, “for the sake of the greatest simplicity consider a swarm of meteorites at rest, and then assume that others from without approach it from all directions, their previous paths being deflected, the question arises whether there will not be at some distance from the centre of the swarm a region in which collisions will be most valid. If we can answer this question in the affirmative, it will follow that some of the meteorites arrested here will begin to move in almost circular orbits round the common centre of gravity.