are regulated and maintained by the Creator of the universe. Both hypotheses, however, agree in one great principle: viz., that the properties of bodies depend upon forces emanating from immovable points (whether substantial or not is of little importance,) of their masses.

§ 8. Now it is no less necessary to test the accuracy of our notions derived from common observation and the first impression of our senses, than to guard against the careless adoption of ill-understood generalizations of the results of experiment in our after progress; and we must be particularly careful to correct the prejudices which are but too apt to infect the mind from this first fountain of our knowledge. With regard to the absolute hardness, for instance, of the ultimate atoms of all kinds of matter:-from the smooth flowing of water, and the gentle breathing of the air, we derive a notion of softness which we can scarcely separate from the constitution of their ultimate particles; but if we take a portion of water in a glass tube, which has been carefully freed from air by boiling, and afterwards hermetically sealed, we shall find, upon agitating it, that the particles strike one another with a ringing sound, quite undistinguishable from that derived from the collision of metals. Such a little apparatus is commonly made, and goes by the name of the water hammer. So with regard to aëriform matter; a mixture of airs may be confined in a mere film of water, which exploding at a lighted taper, will allow the particles of the atmosphere to rush together so suddenly as to produce a loud report, the result of the collision of aëriform atoms.

Again, Newton has drawn a marked distinction between the solidity of "primitive particles" and that of the bodies compounded of them; asserting that the latter are porous. Some solid bodies we at once perceive, when taken in masses, are full of void spaces, or pores; their solid particles appear only to touch each other at particular points, as in a lump of sugar: but we have difficulty in conceiving that the particles of no bodies touch one another, and we seem but to be giving fair credit to the testimony of our senses, in believing that the particles of a solid metal must be in close contact. A little reflection again may correct this prejudice. Many metals may have their density increased; i. e., their particles approximated, by mere hammering or pressure; but to this there is a limit: all solids may have their particles approximated by cold, and to

this no limit has been yet ascertained. Particles which admit of such nearer approach to each other, cannot of course be in close contact; a mass, or an assemblage of such, must be porous. Newton admits that the atoms, even of the densest solid, are placed at distances from one another infinitely greater than their own diameters.

§ 9. If we are liable to such errors of judgment, with regard to matters of direct observation, how careful ought we to be, and how diffident, in embodying notions of things which are wholly beyond the cognizance of our senses;-as of the sizes, forms, or weights of the "primitive particles," or atoms, of matter. We can carry the palpable division of some substances to an almost inconceivable extent, but we have no reason to suppose that we thereby even approximate to their ultimate atomic resolution.

A gallon of water weighs 70,000 grains; one-tenth of a grain of green vitriol does not contain quite one-fiftieth part of a grain of iron; if we dissolve this quantity of the salt in the water, its diffusion throughout its whole mass may be rendered perfectly visible by the addition of a little tincture of the gall-nut; i. e., it may be proved to the eye that each grain of water contains the seventy thousandth part of the fiftieth of a grain of iron; a quantity far too minute to be ascertained by the most delicate instruments, and of which the imagination itself cannot possibly frame an accurate conception: and yet we dare not assert that this inconceivably minute quantity bears any assignable proportion to the weight of a primitive particle.

§ 10. But, to proceed: we are familiar with matter in two states; viz.--at rest; or undergoing a change of position in space: but no particle of matter, nor assemblage, nor compound of particles of matter, possesses within itself a power of changing its existing state of motion or rest; a passive quality which is generally designated by the term INERTIA. This term, however, appears to have been not very happily chosen; inasmuch as it might be taken to designate inactivity with regard to other bodies; which if we were to admit, we should deny that inherent activity by which it is probable that every particle of matter is capable of acting upon every other particle in the universe.

When we are told that a body at rest would for ever remain quiescent, unless it were to receive an impulse from some

extraneous power, the mind at once assents to the proposition; it seems to agree with the results of our experience; the bodies which surround us move not until some power from without sets them in motion: but it requires some reflection in those who, for the first time, consider the subject, to perceive that bodies in motion would for ever continue to move unless they were stopped by some exterior power. Common and careless observation seems to contradict this part of the proposition: we know of no motion on the surface of the earth but what comes to an end, unless extraneous power be applied not only to produce but to maintain it. This prejudice will, however, be corrected by the observation, that all such moving bodies meet with resistance from counteracting forces; and that the heavenly bodies, the only examples with which we are acquainted of unresisted motion, continue constant in their courses. over, we are not without familiar instances of this tendency to a persistence in a state of motion. If we suddenly check motion, in an open vessel filled with a liquid, the rigidity of all the parts of the solid mass brings them simultaneously to a state of rest; but the mobility of the liquid particles of the combination allows of their continuing in motion in their original direction, and the liquid is spilled. The careless setting down of a pail of water, or a cup of tea, may thus, with proper reflection, illustrate an important principle.

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§ 11. Now every particle of matter which we see around us is under the influence of several FORCES exerted upon it both by distant and by adjacent particles; and upon which it acts in return: for the action of one body upon another is always accompanied by a reaction of the latter upon the former, of the same intensity in the opposite direction. To these forces the motions which we observe in some bodies are owing, and upon their balance the apparent state of rest in others is dependent: it is, therefore, of the first importance that we should form as accurate a notion as possible of the meaning of the general term FORCE. The following analysis of the subject, by Sir John Herschel, clearly refers the origin of the idea to the consciousness of each individual; conscience being the highest form of personal experience*.

"We are conscious of a power to move our own limbs, and, by their intervention, other bodies; and this effect is the result * HERSCHEL's Discourse, p. 86.

of a certain inexplicable process which we are aware of, by which we exert force; and even when such exertion produces no visible effect (as when we press our two hands violently together, so as just to oppose each other's effort), we still perceive, by the fatigue and exhaustion, and by the impossibility of maintaining the effort long, that something is going on within us, of which the mind is the agent, and the will the determining cause." In the case of force exerted by the right hand, and met by an equal force from the left, the two acting in opposite directions exactly neutralize each other, and no motion ensues; the effect is pressure; they exactly balance one another, and may be said to be in equilibrio. We recognise a similar power of generating force in other animals by opposition to our own, and as the proximate seat of it is in the muscles, we denominate it muscular or animal force. This force may be communicated to inanimate matter, as when we project a stone from the hand; and that a body thus set in motion carries force with it, we may feel by arresting it in its course. Thus, in stopping a ball at cricket, we experience a shock, and the bones of the hand might even be fractured by the blow if it did not at first yield a little to its course. continued action, as when a stone, which when cast from the hand will inflict a comparatively slight injury, is continuously acted upon by the same arm, by the intermedium of a sling, it will at length be projected with an intensity of action capable of producing very violent effects. We thus learn to regard motion in matter, however produced, as the effect, and indication of force: and force has been defined as "that which is capable of producing motion in matter, or of stopping, or altering its direction when produced."

It may be concentrated in the same mass by

Velocity of motion, or the distance which a body moves in a given time, may, when the mass remains the same, be taken as a measure of mechanical force (as force thus transferred to moving masses of matter is called), and by multiplying the quantity of matter in a body by its velocity, we, in all cases, learn its momentum, or the quantity of force which it is capable of exerting upon other bodies opposed to it. The investigation of the laws of motion constitutes the province of DYNAMICS.

§ 12. This force, which we can exert and communicate at will, and which can be generated by all animal bodies, we may soon become sensible may be opposed by other forces

appertaining to inanimate matter. For example:-in lifting a lump of iron, or lead, or stone, from the ground, we are conscious of the same kind of opposition, and consequent exertion, as when we violently press one hand against the other; and whatever opposes force must be force. If the force which we apply be greater than the opposing force, which we familiarly denominate weight, the mass is raised; but we may so adjust the two, that they may be in equilibrio, and then no motion will be perceptible; but we shall feel its pressure, and, if the mass be considerable, we shall not long be able to maintain the balance, from exhaustion and fatigue, and the mass will fall to the ground.

Again, by pressing with the arm upon a strong spring, or in drawing a bow, we feel another opposing power, which we denominate ELASTICITY. At the first impression, motion may be produced, because muscular force may have the advantage, but we shall soon become conscious of a gradually-increasing opposition, which our utmost exertion will not be able to overcome; our highest degree of force will be balanced; fatigue on our part will put an end to the contest, and the elastic body will return to its former state. If the opposition be suddenly withdrawn, and the bow be allowed to act continuously in its return upon a body which is free to obey the impulse, the bolt or the arrow will have a projectile force communicated to it of considerable intensity, as in the case of the stone from the sling.

§ 13. The very same effect may be produced upon the spring or bow by the action of the weight, as by the power of the arm; and if we were to place the mass of iron upon the former, or suspend it from the latter, the heavy body would be supported, and the elastic body would be bent. Weight may thus be opposed to elasticity, and at a certain point they will balance each other: at this point no motion or visible effect will be produced; but the two forces will not on this account cease to act any more than in the case of the opposite pressure of the two hands; they will exert pressure upon each other. Each of these forces having been separately estimated by our exertion and consciousness of muscular force, when opposed to them, we cannot resist the direct evidence of our senses, that "matter is thus capable of exerting upon matter the same kind of effort as that which we appreciate from a mental impression*.”

* HERSCHEL's Discourse.