CHAPTER II.

THE POSITIVE SCIENCES.

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§ 1. When we pass to the other side of the line of demarcation spoken of in § 9 of the foregoing Dynamic. Chapter, that is to say, to the relation between

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Newton's vis inertia and vis impressa, we find that we are ipso facto considering the relations of different portions of matter inter se, and that these relations, so far as they are relations of force at all, are all of them kinds or modes of ris impressa, are actions and re-actions of vires impressa on each other, or rather on the separate portions of matter in which they reside as modes of motion or tendency to motion.

Now this passage to the other side of the line, that is, to vires impressa exerted by separate physical substances, is a most important turning point in scientific method. It marks the transition from considering the nature of Matter to the consideration of the events in its history, as composing an order of real conditioning. When we think of Matter simply in its nature, we think of it as essentially involving space and force, which together constitute its nature as a real occupant of space, or real existent in the fullest sense of the term, and which give to the whole or any part of it

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both volume and mass. But in so thinking of it we are abstracting entirely from the historical order of its existence, and consequently from the Dynamic. question of the character of its initial state, as for instance, whether it comes into existence single and continuous, or broken up into separate portions; whether homogeneous, or in many specifically different kinds; and therefore also from the question, whether it is originally in a state of rest or of motion; neither affirming nor denying, but simply abstracting from, these and similar alternatives.

But when we proceed to consider it as broken up into separate portions exerting rires impressa on each other, the question of initial state at once confronts us, and with that question we enter upon the consideration of Matter in its historical order as an order of real conditioning. We then enter for the first time upon Positive Physical Science, inasmuch as we have then the actual historical order, or Course of Nature, before us. Not that we begin by attempting the question of the nature of its initial state, or that of the epoch in absolute time when its first genesis took place, or that of the possibility of some form or forms of it being originated de novo from time to time, out of what we conceive to be its real conditions, positively unknowable to ourselves, at any epoch or epochs from the genesis of the first form of it to the present day, or again from the present day onwards into the as yet non-existent future. Far from it. These are not questions to be lightly approached. We have first to make a selection of some class or classes of phenomena, with which

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our enquiry shall begin, and then of some point in the history of the phenomena selected, that is, of some definite event or occurrence in them, which we can begin by examining. It is thus not the nature of Matter or Force simply, but the nature of events or occurrences in their history, that we are then investigating ;-that is to say, the nature of their actions or operations, which is described or defined by the general laws to which they are said to be subject, but which are strictly uniformities which they display, wherever and whenever in the single course of mundane history they or their similars may occur or be repeated. On this previous analytic inquiry into the nature of the operations of matter depends all possibility of ever ascertaining the single course of its actual history, to say nothing of the nature of its initial state, or that of any part or parts of it, which may be or may have been subsequently originated de novo, and not out of pre-existing matter.

Now the simplest form which Positive Physical Science can take has been found to be that in which it deals with the relations and interactions of masses of ponderable matter, that is, matter subject to the Law of Gravitation, taken as homogeneous, but of all degrees of magnitude; and that in three departments, severally devoted to the three states in which ponderable matter is found to be capable of existing, namely, the solid, the liquid, and the gaseous.

In selecting this province with its three departments as the first province of physical science, we are abstracting in the first place from all those intrinsic relations and interactions of masses of

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ponderable matter, upon which the specific properties of different kinds of material substances depend, so constituting them specifically different Dynamic. substances; and from their affinities with each other, and the laws governing their composition and decomposition; as well as from the question whether specific differences of nature are or are not to be found in the ultimate minima of which they are composed; all which considerations belong to the domain of Chemistry.

Secondly we are abstracting from the phenomena of life, and the vital energies in which life consists, which are phenomena and energies manifested in organic substances; since these substances must at any rate be chemical compounds, depending on chemical affinities and reactions.

And thirdly we are abstracting from those modes of force or energy which seem to demand the hypothesis of some etherial, that is to say, some material but imponderable medium, or media, as their vehicle, such as are the energies subserving the transmission of light and radiant heat, and the phenomena of electricity and electromagnetism.

The province of physical science, so distinguished as the first, is Dynamic; and this has two main subdivisions, which, in that department of it which deals with solid matter, are known as Kinetic and Static.1 To these subdivisions of the first department correspond those of Hydrodynamic and Hydrostatic in the one, and those of Aerodynamic and Aerostatic in the other (called also

1 See for the use of these terms, in place of Mechanic, &c., a Treatise on Natural Philosophy. By Sir W. Thomson (Lord Kelvin) and Professor P. G. Tait. Vol. I. Preface, p. vi. Edition of 1879.

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BOOK II. Pneumatic), of the two remaining departments. It is in the first department of this first province Dynamic. of physical science, that the ultimate nature of energy and the ultimate forms in which it operates can be best realised in thought, partly because of the abstract simplicity of the relations studied, and partly because of their greater accessibility to observation and experiment.

With respect to this position at the head of the more concrete physical sciences universally accorded to Dynamic, a few more words must be said. We have seen in the foregoing Chapter, that Force is the doing essential to, and contributing to constitute, Matter, so that without it Matter would not exist; while the other essential constituent of Matter is some portion or portions of three-dimensional space, which force exclusively and adversely occupies. Or again, Matter is active and adverse occupancy of space, and exists by so occupying it. This conception avoids the inconvenience of taking Force as hypostasised per se, which is done, or at any rate not precluded, when it is described, in the most usual way, as "any cause which produces or tends to produce a change in a body's state of rest or motion; "2 whereby "body" also, that is, some or portion of matter, is similarly and necessarily hypostasised apart from force.

mass

But in the present conception of matter as composed of force and the space occupied by it, the distinction between differences of quality and differences of quantity is implicitly contained. Differences in the quality of matter are differences

2 Chambers' Encyclopædia. Article Force. Vol. IV. Edition of 1862.