our other senses have discovered to us different kinds of matter; matter of different sensible properties. Our taste has informed us of sapid and insipid, of acid and sweet, and bitter and saline matter; our organs of smell, of odorous and inodorous bodies; our sight, of blue and green, and black and white, transparent and opaque. It is in the reciprocal action of bodies of such different essences that we trace the existence of our next active force. If we simply dip a piece of metal or glass into water, and withdraw it, we shall have an illustration of it in the wetting of the solid; a portion of the liquid will adhere to it: we can estimate the amount of the force by the weight of the water which may be raised under its influence. If we repeat the experiments before referred to, of placing a few drops of water upon a fine dust, substituting powdered flint glass for resin, we shall be able to contrast this force of heterogeneous adhesion with that of homogeneous cohesion; in the former case the drops of water assumed a spherical form, under the influence of the last power; in the latter case heterogeneous adhesion will overcome homogeneous cohesion; the powdered glass will be wetted, and the water will be absorbed. 22. The last force which it will be necessary to specify, to complete this general view of the forces, is CHYMICAL AFFINITY, the highest degree of heterogeneous attraction. The action of this marvellous power between the ultimate particles of different kinds of matter, constitutes matter of distinct qualities; matter differing in essence from any of its ingredients; matter possessing no sensible property in common with its constituent elements but that of their gravity combined. An inquiry into the laws and results of its action constitutes the chymist's peculiar province. 23. These illustrations may serve the useful purpose of fixing upon the minds of beginners in science some definite notions of the nature of the forces which principally concur to the production of chymical phenomena; the particular laws of their several actions, as far as they are conducive to this result, it is the object of the following pages to examine and illustrate; and for this purpose we will take them nearly in the order in which they have already been notified; and first, with regard to GRAVITY. § 24. The attraction of gravitation is exerted between masses of matter at a distance from each other; it is that sublime power which the astronomer contemplates as extending between all the bodies of the solar system; binding the planets in their orbits, and reaching through space to countless other systems, at distances of which the mind of man strives in vain to form an adequate conception. With this stupendous and all-pervading force the chymist has little concern, except as acting at the surface of the earth, and conferring the property which we call weight. It may be exemplified not only by the fall of a body to the earth, but by the approach towards each other of masses of matter which are free to obey the mutual impulse. Thus, pieces of wood, upon the surface of water, are attracted towards each other or the sides of the containing vessel; and the wrecks of ships are frequently found aggregated together upon the surface of the ocean. A plummet, or weight suspended to a string, is commonly employed to indicate a line directed immediately to the centre of the earth, and which is called a perpendicular. This is the direction of gravitation, which acts in straight lines, when undisturbed, at the surface of the globe. The same plummet, when suspended by the side of the abrupt precipice of a mountain, has been experimentally found to deviate from this perpendicular, having been drawn aside a minute, but measurable quantity, by the gravity of the mass in whose vicinity it has been placed. In common language, we say that a stone or other heavy body falls to the earth: but the influence is reciprocal; the earth is attracted by the stone as the stone is by the earth. The action is directly proportionate to the quantity of matter which each mass contains, and this is the first law of gravity. The quantity of matter in the stone bearing no assignable proportion to the mass of the earth, its influence, though certain, is inappreciable. : 25. Here again it may be necessary to guard against a prejudice derived from careless observation and an inaccurate language lead is proverbially said to be heavy, and a feather to be light; and when a mass of lead and a feather are suffered to fall together to the ground from a height (when abandoned, that is, at the same moment, to the action of gravity), the former reaches the bottom sooner than the latter, gravity appearing to act with greater energy upon one than the other. But if the experiment be made in a space void of other matter, the action upon both will be found to be equal; they will both fall together, and we shall learn that the retardation of the feather, in the first instance, was owing to the large surface which it presented to the resistance of the air. In vacuo the smallest particle of matter and the largest mass fall through equal spaces in equal times. Gravity, in short, is the power of transmitting to every particle of matter a certain velocity absolutely independent of the number of material particles: weight is measured by the effort which must be used to prevent a given mass, or accumulation of particles, from obeying the law of gravity; weight, therefore, depends upon the mass: gravity has no dependance at all upon it. § 26. The intensity of the force of gravity is measured by the velocity of a body moving unresisted under its influence, and is such that in this latitude a body falls, in the first second of time, 16.095 feet, or about 16 feet and one inch. We 27. The second law of gravity is, that bodies attract one another in inverse proportion to the squares of the distances of their centres, to which their action may be referred. Now this is the law which regulates the action of all central forces; of all forces, that is, which emanate from a centre, and spread themselves around that centre; and its generality renders it particularly desirable that we should form an accurate notion of its reason. can best explain it, perhaps, by reference to light. Common experience informs us that the intensity of light decreases with its distance; and in what proportion, a little consideration will enable us to determine. A lighted taper radiates its light in all directions alike; if we imagine such a taper placed in the centre of an opaque globe four feet in diameter, its light will all be dispersed over, and arrested by, the surface of that globe, which will be illuminated with a certain degree of intensity. If we now imagine it removed to the centre of another sphere six feet in diameter, the same light will be spread over the larger surface, which will, of course, be illuminated in a less degree. The distance of the light from the surface of the first sphere would be its radius, or two feet; from the surface of the second, three feet; but the diminutions of the light would not be directly as 2 to 3, or as the mere distance, but as the squares of 2 and 3, or 4 to 9; for it can be geometrically demonstrated that the surfaces of spheres, or any similar sections of spheres, are as the squares of their radii;(1) and it is clear a (1) The principle may be illustrated by experiment in the following way: If, in the annexed diagram, 1 represent a board of a foot square, placed at a certain distance from a light at a, it रु T I that in each case the light is spread over the whole surfaces, and, consequently, diluted in proportion to their surfaces. In the same manner and after the same law, the action of gravity is diluted, if the expression be allowable, upon distant masses. 28. If a gravitating body be freely suspended by a string or rod from a fixed point, it will hang in a vertical position; but if it be moved from that position by a force laterally directed, it will rise in the arc of a circle, of which the point of suspension will be the centre, under the joint action of the moving force and the tension, or cohesion, of the rod or string. When it has reached the point at which its moving force is destroyed by the counteracting forces of gravitation and cohesion, it will immediately begin to descend under the force of gravitation in the same arc; and when it reaches the vertical position, it will have acquired a momentum which would tend to carry it forward in a horizontal direction; the tension of the string will, however, cause it still to move in the circle of 1 which the point of suspension is the centre, and it will, after passing the vertical line, rise through a similar arc on the opposite side, until its velocity is destroyed, which, if no other forces than gravity and its antagonist, cohesion, were to act, would be when it reached a height equal to that from whence it first fell from this it will again descend, and, passing the vertical, rise to the first height; and it would thus continue to oscillate for ever but for counteracting forces. The oscillations of an inva will just shadow a board of two feet square, 2, at double the dis tance; of a board of three feet square, 3, at three times the distance; or one of four feet square, 4, at four times the distance; that is to say, the light which is concentrated upon the first board would be diffused over four times the space, if suffered to fall upon the second; or over nine times the space, upon the third; or sixteen times upon the fourth. The boards may be consid ered (without any appreciable error) as similar segments of spheres, of the radii of their several distances. D |