VI.
ATOMS.
THE word "atom," used by Democritus and Epicurus, Descartes and Gassendi, to signify the smallest possible substance, means rather that which is undivided, and cannot while preserving its nature be divided. This definition implies the notion of individuality, as Cicero says; and, since it prescinds per se from size, a large body might sometimes be called an atom, while a smaller one could not. This being premised, let us consider what an atom is in the system that we are explaining.
As a seal can multiply its own image by impressing any sealing-wax whatsoever, so can the divine archetypal idea multiply the divine image in matter according to the number of impressions that God makes on it. From this it clearly appears that universality belongs to the idea, and that singularity and individuality come from the matter on which it is impressed, as St. Thomas lays down in these often repeated words: Individuatio formae est ex materia, per quam forma contrahitur ad hoc determinatum.{1} Now this impression on matter is in the manner of a virtue derived from God; and the substantial form determines an individual being in its nature. Hence it evidently is not and cannot be made in divided and separated matter, because in that case the form would not be one, but as many as the parts of the disjointed matter. Therefore an individual corporeal substance is continuous -- is an atom. The size of it will depend on the form, which may require more or less extension in the matter, or be itself indifferent as to that. Thus an individual man is a substance in his continuous extension, not an aggregate of minute bodies divided and separated. For otherwise the human soul, which is the substantial form of human beings, would be in itself divided and separated; and instead of one soul there would in fact be as many souls as there would be little bodies of which we should suppose ourselves to be constituted. The same may be said of an individual brute, and of an individual plant, and of any inanimate substance that is individual. So that, if the word atom is to mean an individual corporeal substance, we may call by that name not only a little inanimate substance, but even a plant, a brute or a man. Whence it follows that in the one and the same substance the so-called physical pores, i.e. interstices placed all round each atom, or, as they call it, each corporeal substance of the smallest dimensions, cannot be admitted in the physical system, because they would take away the unity of the individual. But the pores that we do find in corporeal substances do not take away the unity of the subject; and they must be acknowledged, because experience proves that they are.
An atom, understood in the strict sense of the word, requires also indivisibility, not absolute but relative, owing to which the substance called "atom" cannot be divided without ceasing to be what it is. This does not prejudice the indefinite divisibility of matter; for that regards extension as considered by reason of the quantity, not by reason of the nature in which it inheres. The relative indivisibility of matter is especially observable in living things, that will not bear division while retaining the nature of their being, because they have a form that requires a certain organism not to be had in every small quantity of matter. Generally all corporeal substances have a minimum, that cannot be less without ceasing to exist, and therefore may deservedly be called an atom according to the strictest meaning of the word.
But will two individual substances, or atoms, that occupy an equal space, have in themselves an equal quantity of matter? And, without increase of the matter, can the same substance occupy more space than before, or occupy less without diminishing it? This question belongs to the doctrine about the mass and volume of the atom; the volume meaning the place occupied by the atom, or its extension with respect to space, and the said volume being either real or apparent. The place, for instance, which a plant seems to occupy is the apparent, not the real volume; for, although the plant has in all its living substance a true continuation, it yet contains innumerable interstices or pores, in which the substance is not. If it were there, the volume would be not merely apparent, but real. The matter intrinsic to the plant constitutes its mass.
This being laid down, the followers of tbe physical system said that two substances occupying an equal space can have different masses, or a different mass in equal real volumes, and that the same substance may have, without increase of mass, a real volume sometimes more and sometimes less. We remember showing in the year 1878 in the ""Civiltà Cattolica" (Serie x. vol. vi. p. 73) that an argument of Galileo's, to prove the variability of real volumes, had no force; but disapproving of the argument in favour of a thing is very different from disapproving of the opinion itself. Epicurus and Descartes did disapprove of it, for they acknowledged no other density and rarity than what arises from more or less distance between the atoms, and affirmed their extension to be immutable. Cardinal Toledo explains the teaching of Aristotle and of St. Thomas about it in these words: "That which contains little matter in much quantity is called rare. That which contains much matter in little quantity is dense. . . . We have to remember that there is a twofold rarefaction and condensation, proper and improper. Improper rarefaction or condensation is what happens by mere approximation or segregation of parts, without any change or alteration of them, . . . and this [improper rarefaction or condensation] does not take place unless an external body is expelled or introduced. Many among the ancients acknowledged no other than this; but they supposed quite vacant pores in bodies, while we affirm them to be full of a most subtle corporeal substance. Proper rarefaction and condensation is not produced by expelling or introducing an extraneous body, but by an internal change of the subject."{2} Here we have a change of extension. The matter remains the same.
This suggests a natural explanation of the greater or less gravity that substances acquire by change of position. In fact, if part of a solid body be divided and subdivided ever so much by purely mechanical means, it will not be lighter than before; but it really would be so, if properly rarified, so that in rising it made room for the other part of the same body, which was not rarified, and which has more matter under an equal real volume. We say "of the same body," because we must admit that a corporeal substance in a liquid state may be more dense and contain more matter than another corporeal substance in a solid state. This variation of gravity is explained by St. Thomas, according to the doctrine of Aristotle, as follows: "The size of a body is extended or simplified in rarefaction, not by the matter receiving into itself any other thing, but because the matter which first was in potentia to be greater or less becomes actually so; and therefore the substance is not made rare or dense by addition or substraction of extraneous particles, but by the matter itself becoming rare or dense. . . . He [Aristotle] proves his assertion by the effects of the rare and the dense; for the difference between the heavy and the light, the hard and the soft, follows the difference between rarity and density. . . He says therefore that the lightness of bodies is in consequence of their rarity, the heaviness in consequence of their density. And he is right; for the rarity of a substance comes from the matter receiving greater dimensions, and its density is because the matter has less dimensions. Hence, if, of two bodies equal in extension, the one is rare and the other dense, the dense body has the most matter."{3} These considerations deserve to be weighed and examined, not despised.
According to this doctrine it follows that corporeal substances, unless impeded by an extrinsic motive power or other obstacle, will be so disposed around centres of gravity that the more dilated bodies will go further and further from them. Therefore, if all round the earth, for instance, various spaces be supposed in the manner of concentric spherical strata, each corporeal substance would have its own, in proportion to its density, from the densest even to the most subtle ether, whose density is so slight that we can hardly form a conception of it.
And consequently we have to admit that an ethereal and most subtle bodily substance is everywhere diffused in the interplanetary spaces, as the vehicle and subject of the reciprocal operation of the stars and the planets, though these are placed at such enormous distances from each other. Thus without contradicting the indisputable axiom, Non datur actio in distans, we can explain the diffusion of light and heat in agreement with experience. On the contrary, they who, following Epicurus and Descartes, affirm the existence of equally impenetrable atoms unalterable in their extension, are compelled thereby to admit in nature much more of absolute void than of space occupied by corporeal substances. They have to suppose ethereal atoms at a great distance from each other, and thus make inexplicable (according to the true system of irradiation) not alone the extreme rapidity of the propagation of light, but even the fact of its propagation. God, however, Who willed order and unity in the corporeal universe, and made substances to act on other substances in the most various and admirable ways, so ordained that there are substances whose nature it is to dilate and rarify in the highest degree and fill the vast spaces of the heavens, passing between the interstices of bodies almost incredibly small. This is what is meant by the very old adage, Natura abhorret a vacuo. We should be indulgent to the old physicists who explained by this adage such phenomena as the rising of water in curved tubes, and so but the modern physicists have no right to maintain that the Torricellian vacuum refutes the adage. The old adage refers to an absolute vacuum, while the vacuum obtained by art is imperfect and relative.
We have now to pass on from the consideration of corporeal substances, in their individual and absolute being, to consider them in those mutual relations that have their foundation and origin in the seminal causes.
{1} Quodlib. vii. 3.
{2} In IV. Phys., C. 9, Q. 11.
{3} In IV. Phys., lect. 14.