each other. The fact that chemical compounds require definite, and in each case, invariable, proportions of the combining substances, affords strong reason for supposing that there is a limit to the divisibility of all kinds of matter, and that however we may conceive an atom to be divided, it is not really divisible or penetrable by any power in nature. The justness of this conclusion need not, however, be argued at once, and it is introduced here only to represent the material fabric to which the various natural forces are supposed to attach themselves.

But upon the assumption of ultimate atoms, nowhere in absolute contact with each other, we must conclude that space only is continuous. This view has been considered as unsatisfactory in some respects, and it led Boscovich, in the last century, to maintain that apparent matter consists only of "centres of force," or of "unextended nuclei," which would appear to be equivalent to no nuclei at all. Professor Faraday has found it desirable to adopt the hypothesis of Boscovich, in order to account for the conduction of electricity by some substances, and its rejection by others, and he has asked what conception can we form of matter apart from the forces which attach to it? Any conclusion, from a philosopher who has graced so many triumphs of physical research by such rare humility of opinion, must inspire a trust akin to conviction. We cannot, however, delegate the duty of direct inquiry, and in this instance we may learn how much the issue of a question may depend upon the mode of its examination. For we may, with equal reason, ask, what conception can we form of a force apart from the matter upon which it acts? What, indeed, but incessant and indicative changes in the visible forms of matter is there to convince us of the

existence of any force whatever ?

And would it be more metaphysical to deny the existence of force than that of matter? If we are told that materiality is but an illusion, may we not extend the same speculation to force, and imagine that matter moves spontaneously, without force, and that all our sensations of impulse and resistance are psychical only, and unfounded in absolute fact? Such reflections sweep us beyond the range of reason, and confront us with the dysæsthetic, if not impious, conclusion that the universe is a falsehood. This current of thought is not to be trusted, therefore, nor should we embark upon it for a moment except to steer stoutly athwart it, thus to reach the truth which lies but a little beyond. Our only danger in this attempt is that of self-deception, for our course to the penetralia of nature is not always marked by shining facts, and the guiding luminary of reason may be obscured by inconsequential logic. If we do not, therefore, find occasion to mistrust many of our most cherished convictions, we should at least conduct our inquiries in a reverent and hopeful spirit of discovery, rejecting everything like dogmatism for analysis and comparison, and everything like prejudication for warrantable deduction. And here we can have no better example than that of Faraday himself, for it is this spirit which has inspired his whole career as a philosopher. We are only now debating a single inference which he has adopted to meet a supposed difficulty of electrical conduction. Even if space only were continuous, it would be a non sequitur to conclude that space, absolutely void of matter, could constitute a highway for electricity. For it does not follow that electricity, or any other force, can traverse space per se otherwise than from one material point to another. Even interplanetary space is supposed to be occupied

with nuclei of matter forming stepping-stones for light. We may adopt the words of Professor William Thomson : "That there must be a medium forming a continuous material communication throughout space to the remotest visible body is a fundamental assumption in the undulatory theory of light." And the same authority has explained how it is probable that the interplanetary ether is of far greater density than our own expanded atmosphere would be if extended, upon Boyle's law, to the sun.

We are bound, therefore, by no logical necessity to consider Boscovich's hypothesis at all; and however we may elevate ourselves above the ordinary horizon of thought, and with whatever desire to distinguish its latent truth we may approach a given proposition, or attempt to trace a given suggestion to its sources of probability, we can hardly escape the conclusion that the doctrine of non-materiality, of all others, offers the least return for our investigation. Difficult, although not, perhaps, impossible of conception, and subversive of a thousand forms of thought, the supposition of sixty-five visible and tangible forces (many of them gustable and olidible), in the place of as many unmistakably distinctive forms of apparent substance, seems calculated rather to confuse and abridge, than to clarify and extend our knowledge of nature. How can we identify these forces with those of gravitation, cohesion, chemical affinity, and Heat, which offer not even the suggestion of materiality; or, if these be distinct and imperial, how shall we codify the relations which explain their sway over the supposed sixty-five subject forces? The whole notion of non-materiality, indeed, is one of those of which Higgins wrote that "The greatest praise that can be awarded to the hypotheses of motion which have been

invented since the days of Newton, is, that they are the essence of mysticism. There is something so unaccountably perplexing in the conception of their doctrines, that after inquiry the mind is in a very improper state for the reception of a conviction of their accuracy."

In the present essay, therefore, the character and action of Heat will be considered in relation to ultimate, solid, insecable, and impenetrable atoms, of which every description of matter may be supposed to consist. This condition of matter, conjectured by the ancient philosophers, and reasoned upon by Epicurus, was regarded by Newton as at least probable, and the precision and permanence of proportion in which all chemical compounds have since been found to form, afford strong reason, for believing it to be a great fact in nature. The limit of material divisibility has not yet been ascertained, but we have no proof that it is infinite. A single grain, in weight, of metal, in a dilute solution, has been divided, under the microscope, into seventy thousand millions of parts; but this fact does not prove that the division could have been carried a hundred or even ten-fold further. It has been suggested that, with a radius of infinite length, the distance between a curve and its tangent, as measured infinitely near the point of contact, may be infinitely diminished, and hence, it has been argued, matter must be infinitely divisible. The illustration, however, bears no necessary relation to the conclusion. No one can deny that space, or any form of extension, is infinitely divisible, no portion of space being so minute that we cannot conceive it to be still further divided. So, too, we can conceive, and conceive only, of a like divisibility of matter; but in this a mental conception, unsustained by anything like proof, must go for nothing. Nor would

the conception of an indivisible atom, of definite form and weight, deserve serious consideration were it not supported by a chemical fact of prime consequence, and did it not, furthermore, furnish facilities for the pursuit of cosmological inquiries which the opposite hypothesis does not offer.

THE MATERIAL HYPOTHESIS OF HEAT.

When the nature of Heat is said to be embraced by an hypothesis, this term is to be taken, not in its general sense of a proposition conducive to proof, but in that of a theory imagined to account for what is not understood. The broader signification of the term could not, justly, be attached to the hypothesis of substantial Heat, at least, inasmuch as it is wholly imaginary, and although easily conceivable, and indeed plausible, there are but very few facts which impart to it any degree of probability, while there is at least one with which it would appear to be hopelessly irreconcilable. The hypothesis is of a compound nature, it being imagined that, with the single exception of Heat, no form of matter is endowed with repulsive force of its own, and that the particles of Heat (including electricity) exercise no attractive force upon each other. It is further imagined that the particles of Heat are strongly attracted by those of all other substances in nature, every atom in which, other than the supposed atoms of Heat, is assumed to be surrounded by an indefinite number of them, this number being increased or diminished according to changes of temperature, although the diminution can never proceed to the extent of a total privation of Heat. This compound hypothesis thus creates an imaginary substance, assigns to it a force diametrically opposite to every other force at the same time supposed