aggregations, at the same time they do not conflict with such a modification of that hypothesis as assumes the actual magnitude of these units so far reduced as to be practically infinitesimal. They only declare—but this they do in the most emphatic manner—that this reduction must not be so far continued as to make the ultimate atom equal to zero, in the sense of absolute nullity.
On this view, which is by no means a new one, of the ultimate constitution of matter, the units of the so-called chemical elements, even of those having the smallest atomic weights, may themselves be of a relatively high order of aggregation or organization, below which many degrees may exist in which the molecules are too minute to form bodies which the senses can in any manner detect. The interstellar ether may be explained as constituting one of the highest of these degrees, yet not high enough to form matter such as to be visibly subject to the law of gravitation. The nebulæ present the evidence of the lowest form of such so-called "ponderable matter," and these may be supposed to be the result of a gradual development resulting from the successive recompounding of the molecular aggregates, until they finally acquire a certain influence over one another and tend to molar aggregation, forming the nebular masses. At the outset these aggregates may be supposed to be entirely homogeneous, consisting wholly of molecules of the same degree of aggregation, but they soon differentiate into several distinct kinds of matter. These are the gases which the spectroscope reveals in some of the nebulæ. They have molecules of low atomic weights and remain gaseous at all temperatures artificially producible. This process of evolution, which is the same which we have seen to go on in all the well-known forms of matter, would seem also to continue throughout the history of the nebulæ and the organization of resultant planetary systems, developing many additional forms of matter, likewise characterized by the increasing mass of their molecules.
What the properties of those molecular aggregates may be whose activities can not be revealed to sense, is of course unknown. Conjecture even as to the probable number of degrees of aggregation from the ultimate atom to the supposed atom of hydrogen would of course be idle. But that such forms exist far down upon this inaccessible plane, having definite shapes, sizes, and activities, we are strongly led to assume, both by the facts already stated and by others presently to be set forth.
Passing over these lower stages, therefore, whose study belongs to the future of human science, or to possible beings endowed with finer faculties, and which may be said to belong to the domain of transcendental chemistry, we finally arrive at a class of aggregates of great, stability, but which, though still so minute that they can only be perceived when accumulated into masses, have nevertheless been studied in their free state by means of the various phenomena to
