and uncommon in terms of that which is ordinary and common. But that which is most obvious to the senses is both the earliest and most persistent presence in consciousness, and thus receives the stamp of the greatest familiarity. Now, the most obtrusive form of matter is the solid, and for this reason it is that form which is first cognized by the infant intellect of mankind, and thus serves as the basis for the subsequent recognition of other forms. Accordingly we find that, on the early stages of human history, the solid alone was apprehended as material. It was long before even atmospheric air, obtrusive as it was in wind and storm, came to be known as a form of matter. To this day words signifying wind or breath—animus, spiritus, geist, ghost, etc.—are the terms denoting that which is the fundamental correlate of matter, even in the languages of civilized nations. And it is very questionable whether either the ancient philosophers or the mediæval alchemists distinctly apprehended any aëriform substance, other than atmospheric air, as material. It is certain that up to the time of Van Helmont, in the latter part of the sixteenth and the first decades of the seventeenth century, aëriform matter was not the subject of sustained scientific investigation.
It is obvious, then, that, while the progress of evolution in Nature is from the aëriform to the solid state of matter, the progress of the evolution of knowledge in the minds of men was conversely from the solid to the aëriform; and, as a consequence, the aëriform or gaseous state came to be apprehended as a mere modification of solidity. For the same reason, the first form of material action which was apprehended by the dawning intellect of man was the interaction between solids—mechanical interaction—and from this, again, it followed that the difference between the solid and the gas was apprehended as a mere difference of distance between the solid particles, as produced by mechanical motion.
Again: familiarity, in the minds of ordinary men, is universally confounded with simplicity. And, the explanation of a phenomenon consisting, as we have seen, in an exhibition of its genesis from its simplest beginnings, the mind, in its attempts to explain the gaseous form, naturally retraces the steps in the evolution of its ideas concerning matter—of its concepts of matter—back to the earliest, most familiar, and therefore apparently simplest form in which matter was and is apprehended, and assumes the solid particle, the atom, as the ultimate fact, as the primary element for all representation and conception of material existence.
This is not the place to develop the important consequences which flow from the total subversion of the prevailing concepts respecting the constitution of matter that, in my judgment, is inevitable. When it comes to be fully realized that an aëriform body is not a group of absolute solids, but is elastic to the core; that a gas is a gas throughout, and in its very essence; that in the simplest states of matter there
