I had hopes, when I had got thus far, to have been enabled to close this long and tedious preliminary investigation, and, for the present at least, to have exonerated you from any thing further on so dry a topic.. But on looking over my letter, I find a few explanatory words may be necessary on that property in nature which, for precision and conveniency, had the appellation given to it by Newton of the vis inertia. From what has already been said on the subject, you will have thought it not difficult to conceive his meaning. Ingenious men, however, have found fault with the expression. It attributes to body, say they, a power or energy, a force, a something which appears as inconsistent as if it gave the power of light to darkness, or of sight to the blind. For what is power or energy? Is it not a disposition to act, either spontaneously, or in consequence of some impression? In Newton's reasoning there is a state of inactivity endued with action, or a motionless motion. A state of rest can only be a negative to that of motion; and when that ceases, this must always obtain. "Mobility in atoms," remarks Mr. Jones, "is nothing more than a capacity of being moved by any mechanical force applied to them; not that they are endued with any internal power of moving themselves.

VOL. I.

I

* Philosophy of the Elements.

selves. If we should say, that matter is by nature inert and resisting, and indifferent to motion, and then should proceed to give the same matter a disposition to move without some external force, we should invest it at once with power and impotence, with sluggishness and activity, which cannot both be predicated of it, without contradiction. The trumpet has a capacity of sounding, but never till it is sounded of itself it is dead and silent, and, as long as left to itself, must always remain so. Matter, in like manner, hath a capacity of motion, not an ability to move. Let us suppose the present laws of nature all suspended, and all matter annihilated except one single atom; we all confess that this atom can have no inclination to move, but must remain absolutely at rest; and all this from its own inert nature. If we introduce another atom, they will both be at rest; for the properties of matter are not changed by having two atoms instead of one. To invest matter with any innate powers or self inclinations to motion, is as contrary to the real nature of matter, as to suppose that all trumpets are born with lips and lungs, and breath of their own: it is to make the world an animal, and all the parts of it animated, as the Stoics of old did; it is to resolve all attractions and repulsions into an animation of matter, and

suppose

suppose them to be the work of certain souls and spirits resident in all natural bodies, and commissioned to act in the several departments of nature."

Notwithstanding these strong objections, and various similar ones, which were started at the very birth of the doctrine, Newton, and many of the deepest philosophers since his time, have maintained, that there is not any thing in physics better settled than this vis inertia or passive force, by which matter always continues of itself in the state in which it is, and never changes that state, but in proportion to a contrary power acting upon it; so that the very same force which is necessary to give any certain velocity to any certain quantity of matter at rest, is always exactly requisite to reduce the same quantity of matter, from the same degree of velocity, to a state of rest again. "This vis inertia is always proportional," say they, "to the quantity of matter, whether at rest or in motion, and is never transferred from one body to another. Without this vis, the least force would give ve. locity to the greatest quantity of matter at rest; and the greatest quantity of matter in velocity of motion, would be stopped by the least force, without any the least shock. So that, properly speaking, all action and re-action, all impulse and resistance, are nothing but this vis inertia

in different circumstances; and, indeed, all force is only a different modification of rest or motion." In regard to the expression itself, fastidiousness. alone can find fault with it; and this short explanation will, I hope, be sufficient to shew you that it is neither erroneous nor unphilosophical. Newton having asserted the inactivity of matter, it was natural for him to make use of a word not any respect applicable to activity. The doctrine he had to establish was new, and required The vis inertia, therefore, which is a vis insita, or innate tendency, struck him.He applied it to the idea; and it has stood the test of the most enlightened understandings.

in

a new term.

In like manner, the occult qualities of the ancients, which Newton has been accused of adhering to, by establishing the probability of the doctrine of attraction, were entirely in the imaginations of his opponents, and not in the principles of his doctrines. By the terms attractive and repulsive forces, he never meant to express inherent forces in the molecules of matter. Body, he well knew, cannot act where body is not. But let us suppose the word attraction to be suppressed, can we deny the planetary motion? Or can angry criticism destroy the sublimity of Newton's theory, or the correctness of his calculations?

LET

LETTER XIV.

THE elements are the original component parts of bodies, or they are those into which they are ultimately resolvable. The ancients in general agreed that the elements were four in num, ber, yn, vdwę, nug, aga; earth, water, fire, and air. Aristotle admitted these four elements, and acknowledged they were sufficient to form all sublunary bodies; yet from the beauty of the firmament he was induced to suspect the heavens to be formed of finer materials; and therefore supposed a sort of fifth element, the quintessence of the other four, for their peculiar construction. But this notion was adopted by very few, and the four elements were, for ages, received as the basis of all natural bodies.Many modern chymists, however, have disputed the doctrine of the four elements. Notwithstanding which, it does not appear that they have been able to establish any in their stead, which will stand the test of a rigid examination; or concerning which they can generally agree among themselves. The truth is, we cannot always