identical with, that which is now presented by a large number of irresolvable nebulæ whose spectra show them to be composed of gaseous matter in an incandescent state. This gaseous or nebulous condition, though exceedingly rare relatively to the solid forms of matter familiar to us, is nevertheless a state of a high degree of aggregation as compared with the forms of matter by which it is surrounded and with its wholly unaggregated state. Before the operations which may be designated as molar can commence, a degree of aggregation must be reached far exceeding that which exists in those molecules which are the vehicles of luminiferous radiations. The particles constituting the ethereal matter of interstellar space must be supposed to be so minute and relatively far separated as not to exert any appreciable influence upon one another tending to produce molar motion or organization; a condition which is explained on the same grounds as the fact that one system in space exerts no appreciable influence upon another system.
If the so-called chemical elements are simply so many stable molecular aggregates, whose differences are due to different modes and degrees of aggregation, then the gases of our earth are simply the most diffused state in which masses of these aggregates can be obtained. A gas is a diffused mass of homogeneous molecules, and this definition is as true of the compound gases, steam, carbonic acid, or vapor of alcohol, as it is of the simple ones, such as hydrogen, nitrogen, or vapor of mercury. It might, then, be naturally supposed that the nebulæ would contain a number of such gases, and as it is scarcely to be presumed that all the modes of forming stable aggregates are represented on our planet, so, in addition to some of those found here, it is reasonable to expect that nebulæ will contain some not known to us. In so far as the spectroscope—to which, indeed, we owe all our positive evidence of the existence of true nebulæ—is able to inform us, this view is confirmed. Two of our commonest gases, hydrogen and nitrogen, have been identified in nebulæ, while a third has been discovered which has not yet been identified with any known element.
Every modification of the nebular hypothesis yet put forth has been compelled to assume that the original nebulous mass must be in an incandescent state. Certain it is that all visible nebulæ are selfluminous. But this is a condition of their visibility. It can not be known how many may exist which have not yet reached this state, and are, therefore, invisible. It does not seem necessary to suppose that the contraction of a nebulous mass is either due to, or requires, a high temperature. No reason exists why cold particles may not become collected into a diffused mass. The inherent motions of these particles are not increased or diminished. But, these motions remaining the same, their circuits of motion are reduced, the frequency of contact is increased, and heat and light are evolved from the friction. The tendency of all matter under the law of gravitation, considered