The idea of Descartes on the essence of matter was carried by his followers to the full extent of using matter as a synonyme for exten sion. Le Grand says that a vessel filled with gold has not more matter than one filled with water. There is more weight, he says, more hard ness, &c., but not therefore more matter ; for the essence of matter is not in weight, nor in hardness, &c., but in extension. And he objects to the adage that "Nature abhors a vacuum," because he considers such an assertion merely to amount to saying that Nature abhors a contradiction in terms. Newton (` Principia,' book iii., prop. 6, cor.'4) expresses his opinion of the vacuum question in this way : " If all the solid particles of bodies are of the same density, so that rarefaction cannot take place without the creation of pores, there must be a vacuum." Since matter is of different density in different substances, and since the same substance may be compressed into smaller space or expanded into larger, it must either be that the solid particles are contracted or expanded, or that vacuous pores exist. This alternative does not do much. A person trained in the sciences as they now exist, thinks the idea of solid matter (that is, entirely solid, without any vacuum) being compressed into more solid matter, to be most incon gruous and improbable ; but impressions derived from habits are not arguments. The strong part of the Newtonian argument arises how ever from the results of the planetary theory. These celestial bodies have moved, during two thousand years of recorded observations, with exactly the same mean motions as at present, which they could not have done if they had moved in a medium of any sensible resistance. If then the celestial spaces be full of matter, it is matter of such a degree of tenuity that two thousand years is not enough to make it show any visible effect in altering the planetary motions. But again, though this argument has, almost up to the present time, induced astro nomers to suspect an absolute vacuum, yet very recently the feather has shown a resistance which was not manifest against the guinea. A comet has been strongly suspected—all but proved—to be undergoing precisely the same sort of change in its mean motion which it is known would result from a resisting medium. [COMETS, coL 68.] The undu latory theory of light, moreover, which is now pretty generally received, supposes the whole of the celestial spaces to be filled with the lumini ferous ether. The astronomical argument, therefore, in favour of absolute vacuum has fallen ; but the views of the constitution of matter which have grown with the rise of the molecular sciences of chemistry, light, heat, electricity, &c., have supplied its place with much more effect. We cannot enter into the various probabilities in favour of the molecular theory, which supposes matter to be atomic, the atoms being perhaps separated by distances which are many times their own diameters. If any one were to assert that the densest substance has in it many millions of times more of vacuity than of solid matter, the assertion could not be disproved, nor even shown to be improbable. "There are difficulties," said Dr. Johnson, " about a plenum, and there are difficulties about a vacuum, but one of them must be true ;" that is, either all space is full of matter, or there are parts of space which have no matter. The alternative is undeniable, and the inference to which the modern phi losophy would give the greatest probability is, that all space is full of matter in the common sense of the word, but really occupied by par ticles of matter with vacuous interstices ; showing all degrees of density, from that of the ether of light, which is wholly unappreciable, to that of hammered platinum, which is twenty-two times as heavy as water.
I'robably the manner in which the reader is most familiar with the use of our leading word is in connexion with what he may have seen written on the maxim which we have already quoted—" Nature abhors a vacuum ;" a doctrine which, though common among the followers of Aristotle, must not, any more than many others, be therefore taken as emanating from that philosopher himself. This is usually cited as
a proof of the puerility of the ancient and middle philosophy—we think, somewhat unjustly. The personification of Nature is common to all times, and we are in the habit of saying that Nature exhibits phenomena, conceals her operations, uses the simplest means, &c. Now Nature may as well abhor, as exhibit, conceal, or employ ; and where intelligence is understood, all who use the word Nature mean the God of Nature : while when the mere operations are referred to, Nature is only the personification of the aollective body of second causes. As the statement of a fact, it is true : Nature does, to the best of our knowledge, abhor a vacuum; she (if we may personify her) never suffers it to exist to the extent of allowing any apace which is perceptible to our senses to be vacuous. But if the adage were meant to supply a reason for the fact, those who used it were deceiving themselves, but not so that the most of those who would laugh at them would have any reason in their mirth. It is the error of every period to use words expressive of a fact observed in the sense of assignment of a reason for that fact ; and the centuries which have always been ready with their fluids to stand for the causes of heat, electricity, magnetism, &c., shduld not be too hard upon the pre ceding ages, which put the feelings of nature in the place which they rather prefer to occupy by hypothetical gases. The very word attraction [ATTRACTION], in the sense generally assigned to it,. is pre cisely of the same nature as the natural abhorrence of the Aristotelians : namely, a word invented to supply the place of a cause. Those who can use the former word in a really philosophical sense are those who can see that some of the ancients may have done the same with the latter.
" The question of the existence of vacuum, in its strict and absolute sense," to repeat the designation given in the preceding portion of this article, which is reprinted as it originally appeared, on account of its historical and philosophical value, is inseparable from that of the nature of space. If space, as suggested in a former article [PHYSICAL FORCES, CORRELATION or, col. 496], be "the extension of material substance, the resultant of its dimensions, and mere consequence of its existence," an absolute vacuum is in the nature of things impos sible. But the admission of the existence of space distinct from matter is equivalent to affirming the existence of an absolute vacuum. Space distinct from matter is nothing else. This subject, however, resembles others of what may be termed transcendental natural philosophy, such as the (alleged) infinite divisibility of matter or of space, the absolute-zero of heat, &c. The affirmative of each is purely imaginary, being something which is mentally conceived to be abstractedly and intrinsically possible, without any reference to known physical facts, which are gratuitously assumed, not in reality to define and limit the subjects, but to depend altogether, in relation to them, on the necessary imperfections of the senses and of our finite condition. But neither by observation or experiment, nor by mathe matical reasoning from either do we know anything about space distinct from matter, about the infinite division of matter, or about the existence of anything but at some temperature or amount of heat in the state or condition in which it causes expansion. (The calculations which have been made as to the number of thermometric degrees between some known temperature and the supposed absolute zero are entirely nugatory, and unworthy of attention. There is no more reason to believe in the existence of an absolute zero than in absolute rest, or in a limit to space, or than to believe, for philosophical reasons, in the cessation of pheno mena, or in the beginning or the end of time.) And these three subjects —the alleged absolute vacuum, infinite division of matter or of space, and absolute zero or its converse—are as inseparably connected in mental conception, as are the physical types of which they are abstractions in observed fact.