ascription of inertia to matter, he proceeds to enumerate other properties, viz., "extension, impenetrability, figure, and color." Certainly it has not been proved that the above properties are all essential to the existence of matter, nor that they are all the properties which matter possesses. Evidently, matter cannot be defined by an enumeration of its properties unless we know what are its essential properties, being thereby able to exclude every non-essential and to include every essential property. This knowledge we do not at present possess, as all admit. Consequently, Dr. Cook no doubt concurs in the opinion that a logical definition has not been given, and cannot be; and that the descriptive phrases employed in the place of a definition are defective and unsatisfactory. That we are unable to enumerate the essential properties of matter will appear in the chapter following.

CHAPTER XII.

MATTER; ITS PROPERTIES.

OF matter, then, we have, as we have seen, no definition which will bear scrutiny. We cannot say of what genus it is a species. Relatively, it is a summum genus, and is therefore extremely difficult to define. We may affirm that it is not force, not life, not mind, not spirit; still we have no knowledge of its existence except as associated with one or more of these. Of its essence we know nothing. Science has not as yet shown itself competent to the task of enumerating its necessary qualities. Its properties are usually classified as follows:

1. Essential properties, or such as are supposed to be necessary to our conception of matter, and to suffice to . define it; viz., impenetrability and extension.

2. Accessory properties, or such as, though not essential, are supposed to be shared by all bodies; viz., divisibility, porosity, compressibility, dilatability, elasticity, mobility, and inertia.

3. Specific properties, or such as belong to matter under certain forms; viz., solidity, fluidity, tenacity, transparency, color, figure, etc., e. g., the properties of oxygen, of iron, of vegetable substances, of animal matter, etc.

It would be hazardous to affirm that these properties, or indeed any definite number of them, are ultimate facts. The ultimate qualities are no doubt much fewer.

Of course particular properties cannot be said to be properties of matter; they are merely properties of certain aggregations of matter. Solidity and figure may be said to be qualities of granite, of iron, etc. Fluidity is a quality of mercury, of water, of milk, etc. Tenacity may be designated a quality of oak, of tin, of zinc, etc. Transparency is a property of glass, of clear water, etc. Neither of the five, however, can be said to be a quality of matter, for matter can unquestionably exist in forms which do not, and cannot reveal these properties, nor indeed any one of them. No one pretends that color is an essential quality. Of the accessory properties, as is evident, elasticity, dilatability, compressibility, divisibility, and porosity cannot be properties of an atom, which is none the less matter on that account. If an atom has pores, or is elastic, or can be compressed, or can be dilated, then it can be divided. If it can be divided, it is not what it is defined to be, the smallest possible portion, incapable of further division. And if an atom cannot have these properties, then they are not essential to matter. They can belong to it only on the hypothesis that it is infinitely divisible, a single particle being capable of pervading immensity. But if it is infinitely divisible, space must also be infinitely divisible. Unless we are prepared to believe that both are infinitely divisible-which science does not sanction,-we are forced to concede that elasticity, porosity, compressibility, dilatability, and divisibility are not properties of matter, though they are properties of aggregations thereof. Mobility, or the property in virtue of which a body may change its position, few, if any, would be disposed to regard as an essential property. Indeed, absolute rest is a thing unknown. Everything is in motion; perhaps is subject to several motions at the same instant.

The mountain-rock and the forest are moving with the earth on its axis, are journeying around the sun, are swept through space in conjunction with the solar system; perhaps, as some scientists assure us, their atoms are also in ceaseless motion, those of the densest rock and those of the hardest tree producing, it may be, a ceaseless hum by their infinitesimal movements. Rest and motion are relative terms. Mobility, a purely negative quality, is accordingly not generally regarded as an attribute of matter. If attraction, repulsion, and polarity are properties, they are one property variously viewed.

Nor can it be proved that inertia-the inability of a body to set itself in motion when at rest, or to cease moving when in motion-is an essential property. As already intimated, perhaps it is more accurate to say that not inertia, but motion is an attribute of matter, no material substance being ever at absolute rest. Even on this hypothesis, it should be borne in mind that it is one thing to say that it does not cease moving, so far as man can discern, and another thing to say that it cannot cease moving. If matter is incapable of changing its state of rest, then, seemingly at least, neither attraction nor repulsion are among its essential attributes; if it is incapable of changing its state of motion, then, apparently, there can be no such thing as an equilibrium of forces. We are not warranted, however, in saying that the forces of an atom may not so accurately counterbalance each other as to leave it at rest, provided it were not subject to the forces of other atoms. Unless an atom has the attribute of motion, so that it would move though it were the only material substance in the universe, motion cannot be said to be an attribute of matter. Unless rest can be produced in a universe in

which atoms mutually influence each other, tending to produce motion, then absolute rest cannot be said to be an attribute of aggregated molecules-the molecules of every individual mass of matter must be in motion inter se, and must tend to produce motion in every other adjacent body.

We may assume that if matter were at rest it would remain so eternally unless set in motion by some power from without; and that if it were in motion it would continue moving forever unless stopped by some external agencies; that is, we may assume that inertia is one of its essential properties. This has not been proved, however. It is a purely gratuitous assumption. We have no knowledge of it at absolute rest. We have no testimony which warrants us in asserting that it is incapable of originating motion. It may be-probably is. We can say no more. Accordingly, eminent scientists concede that inertia is not an essential property. Ganot defines it as "a purely negative property," and of course admits that it is not a necessary attribute. Though matter may exist, it is true, under forms which are incapable of being subjected to human observation, or even to scientific experiment, it is nevertheless unscientific to ascribe to it a property which has not been proved to belong to any of its forms yet brought under scientific investigation. Though Prof. Tyndall may ask, "Who will set limits to the possible play of atoms in a cooling planet?" and may regard all life as once "latent in a fiery cloud"; and though the admission that molecular activity may possibly be an invariable attribute of aggregated matter may seem like a concession to materialism, we are indisposed to regard inertia as an essential property. The concession can do materialism no good, for Prof. Tyndall himself admits that "molecular motion explains nothing. . . The pas