Page:Popular Science Monthly Volume 18.djvu/546

530 greater or less number of wholly undecomposable aggregates, serving as the primary basis of all tangible substances.

It must be expected, however, that these elements will possess all degrees of capacity for manifesting their presence, and that while some will stand out boldly, cohere in vast masses, like iron, for example, and in various ways render themselves obvious and obtrusive, others will be ever hugging the confines of the imperceptible, and, like ozone, will perpetually evade the full scrutiny of science. To this latter class also belongs the substance which emits the green ray of the solar spectrum, which has already led eminent chemists to conjecture that it may be of simpler constitution than any recognized element, if not the primary form of matter.

Setting out with the elements, regarded as aggregates of a comparatively high order and stable organization, but differing from one another in form, size, and molecular activities, as widely as the masses they form differ in properties, the problem of the formation of the higher orders of aggregates becomes comparatively simple. We find ourselves already in the domain of experimental science where the more or less completely demonstrated laws of chemistry and molecular physics lead us up to the formation of the various inorganic and organic forms of matter. The constitution of the various substances found upon the earth is readily determined by decomposing them and weighing their constituents. The precise conditions, however, which have resulted in their formation as we find them and brought about the existing state of things in the universe, are not so easily determined, and for this purpose a further extension of the general law of material aggregation is required.

The study of the earth's crust clearly indicates that very different conditions have existed upon it in the remote past from those which we now find. The facts as a whole prove beyond a doubt that our globe has once been in a state both of greater or less liquidity and also of great heat, and that, as its surface has cooled down, the solid parts, to which alone we have access, have been formed, though to what depth these extend we are still ignorant. But, notwithstanding certain doubts which have from time to time been cast upon it, the theory which was very early advanced as most in harmony with the probable history of the planet, and according to which the cooling process has not yet reached the great interior, which is therefore still in a heated and molten, condition, still furnishes, perhaps, the most rational explanation yet made of the phenomena which the earth presents, and also best satisfies the a priori requirements.

It is now generally believed that the present condition of our earth, and also that of the entire solar system, has been the result of a cosmical process of development by which its matter, unchanged in quantity, has been slowly condensed from a diffused nebulous state, occupying enormously increased space—a condition analogous to, if not