the skin affords them but a very thin covering, no particles of matter so small as those which form light, sound, or odours, can affect them; and the sense of feeling gives us no sensation of distant objects, but of those only whose contact is immediate. It appears, therefore, that the difference between our senses is occasioned by the more or less exterior position of the nerves, and of their greater or smaller quantity in the different organs. It is for this reason that a nerve, when irritated by a stroke, or uncovered by a wound, gives us often the sensation of light, without the assistance, of the eye; and from the same cause we often experience sensations of sound, though the ear be not affected by any thing exterior. When the particles of luminous or sonorous matters are re-united in great quantities, they form a kind of solid body, that produces different kinds of sensations, which appear not to have any relation with the first. The particles which compose light being collected in great quantities, affect not only the eyes but also the nervous parts of the skin, and produce the sensation of heat, which is a sentiment, different from the first, though originating from the same cause. Heat, then, is a sensation arising from a contact with light, which acts as a solid body, or as a mass of matter in motion. The action of light, like other matters in motion, is evident when we expose, light bodies to the focus of a burning glass; the action of the light communicates before even it heats them, a motion by which they are disturbed and displaced. Heat, then, acts as solid bodies act upon each other, since it is capable of displacing light matters, and communicating to them a movement of impulsion. The like happens when the sonorous particles are collected in great quantities; they produce sensible agitation, which is very different from the action of sound upon the ear. Any violent explosion, as a loud clap of thunder, shakes us, and communicates a kind of trembling to all the neighbouring bodies. Sound then also acts as a solid body, for it is not the agitation of the air which causes this tremulous motion, since even at that time we do not remark that it is accompanied with the wind; besides, however strong the wind may be, it never produces such violent agitations. It is by this action of the sonorous particles that a cord in vibration sets the next in motion; and we ourselves feel, when the noise is violent, a kind of fluttering very different from the sensation of sound by the ear, although it be an effect of the same cause.