relatively to each other, which come into constant collision. Their velocity is such that at each collision heat and light are produced. In the language of our progenitors, who in the absence of matches used flint and steel, they 'strike fire' against each other. The idea of such a process originated with Prof. P. G. Tait, in an attempt to explain the tail of a comet, but it was elaborated and developed by Mr. Lockyer in his work on the 'Meteoritic Theory.'
The objections to this theory seem insuperable. A velocity so great, at such a distance from the center of the nebulæ, would be incompatible with the extreme tenuity of these objects. Every time that two meteors came into collision they would lose velocity, and, therefore, if the mass was sufficient to hold them from flying through space, would rapidly fall toward a common center. The amount of light produced by the collision of two such objects is only a minute fraction of the energy lost. The meteors which fall on the earth are mostly of iron, and, were the theory true, numerous lines of iron should be most conspicuous in the spectrum. But the fact is that in the great number of these objects there is but a single bright line, which does not seem to correspond to the line of any known substance. The supposed matter which produces it has, therefore, been called nebulum.
