received upon the occasion by the Royal Society. The conclusion from the whole is plain enough: therefore, instead of making any remarks of my own upon it, I shall add the more weighty and ingenuous reflection of the celebrated author himself" what may be said to the propagation of sound

through a medium, according to the re"ceived theory of the air, above 300,000 "times rarer than what we breathe, and next "to a vacuum, I must confess, I know " not."

CHAP. II.

Some positive Proofs, that a Medium, endued with very great Force, is present between the Interstices of grosser Bodies, and in other Spaces usually called Vacuums.

L

EAVING then the celestial vacuum to

be proved by some future arguments, more unexceptionable than any hitherto advanced;

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*Phil. Transact. No. 360, p. 978. This account is preceded by several others, leading to the same conclusion, in the 2d vol. of Mr. Motte's continuation of Lowthorp's abridgment, p. 138.

vanced; we must descend to examine the state of some lesser vacuums; whether made by the help of art and machinery, or left by nature itself in the interstitial vacuities of solid or fluid bodies. That none of these can, with any philosophical strictness, be termed vacuums, though we are sometimes. obliged, for the sake of distinction, to call them such in common discourse, will be clear from the following considerations.

Suppose a receiver (plate III. fig. 1.) to be placed upon an air-pump, and well evacuated. Into the plate at the top of it, a cylinder of wood or metal is inserted, and made to communicate with an electrical machine in motion. If the room be dark, a stream of fire will be seen to issue from the lower end of the electrised cylinder, and go straight down to the plate at the bottom. But if any person applies his finger to the side of the glass, as in fig. 11. the stream will receive a new direction, and he bent to that part of the glass to which the finger is applied. The better the receiver is cleared of its air, the better this effect will be found to answer.

In the opinion of some spectators, prepossessed with the notion of qualities, this

might pass for a good proof, that the finger attracts the fire. Nevertheless, that some subtile medium, though invisible, is present to divert the course of the fire, may thus be proved. It may be previously learnt from some other experiments, that the electrical fluid is acted upon according to mechanical laws. Thus, for example, the resistance of the air being much greater upon the spherical surface of a large cannon bullet, than upon the point of an iron rod; the fire will be confined within the bullet, while it escapes with ease from the point of the rod. Where the resistance of the air is removed from one part of a body while it acts upon another, the electrical stream will go off at that part from which the resistance is removed, without any regard to the figure of the body. In short, it is found, from these plain and intelligible cases, that this fluid acts as it happens to be acted upon, and is impelled to that side where it meets with the least degree of pressure. As the medium within the pores of solid bodies is known to resist it less than any other we are acquainted with, for this reason it is driven toward the finger. But then if there be no medium within the glass, the stream will be equally affected on every

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