us round again somewhat nearer to the old

exploded Aristotelian doctrine of gravity and levity in the elements. The late discoveries in electricity have introduced us to the knowledge of an element which does not gravitate, at least not in its natural or volatile state; neither does it seem very clear that pure air, divested of all humidity and terrestrial adulterations,, has any perceptible gravity. Boerhaave was much in doubt about it, and few men ever pursued experimental science with so much ardour and

judgment as he did. The ancients knew that leathern bottles, blown up with air, were heavier than when lax and empty; but they imputed the apparent weight of such air either to the breath, or to some other accidental humidity, and therefore still persisted to assert its natural levity.

There is another doctrine of the ancients, which the moderns cannot controvert, and, now they have such a field of experiments before them, it is to be hoped they will improve upon it: it is likewise of great antiquity amongst the Greeks; for I find it in Ocellus Lucanus, one of their earliest authors, from whom I suppose it to have been borrowed by Cicero; namely, that two of

the

the elements are active, and the other two passive. Fire and air have the powers of motion; water and earth are only capable of receiving their impressions of which doctrine we have an obvious instance in the body of man, which hath life and motion so long as air and fire maintain their proper stations, and perform their proper offices in it; and when they have left it, it is nothing but an inactive mass of earth and water, falling quickly into dissolution.

182

DISCOURSE IV.

Of Fire, its Properties and Effects,

On the several Kinds of Fire.

IRE is that subtile fluid which remains

FIRE

when the air is withdrawn: it is present within the pores of all bodies, as well as in the free spaces of the atmosphere and the heavens; and it affects us with light, heat, and a sensible force. The eyes distinguish it by its splendour, the blood grows hot with its motion, and the muscles are sensible of its impulse.

Fire is commonly divided into three sorts, solar, culinary, and elementary. The solar is that fire which resides in the orb of the sun, as in its reservoir or fountain, and proceeds from it in the form of light. The culinary is that fire which is kindled upon earth by any artificial means, and burns in any sort of fuel. The elementary is that subtile fluid which resides constantly in all gross bo

dies, and is not necessarily distinguished by its heat as culinary fire, nor by its light as the solar or sidereal fire, but is known by other effects, even in a cold invisible state.

If it is true that natural effects are not to be ascribed to many different means or agents, where fewer will suffice, these three ought to be one and the same fluid, because they have the same properties and the same effects. The solar fire will burn in fuel, and act on solid matter with greater effect than the most violent fire of a furnace; the culinary fire will promote vegetation, and ripen fruits as the sun does; the elementary will light a candle, and fire gunpowder, as the culinary, and will afford a spectrum of the seven primordial colours, in common with the solar rays, or the light of an ordinary fire, and will also throw metals into fusion with a violent scorching heat.

They agree very nearly in their properties and effects, but differ as to the places of their residence: the one residing in the sun, the other in the earth, the other in burning fuel. They are likewise agitated with a different sort of motion; the solar matter, or light, moves in right lines; the culinary vibrates and tends naturally upwards; the elementary

N 4

mentary presses in with a shock to restore an equilibrium, and is diffused in all directions, instead of mounting upwards as the culinary, or being reflected according to the angle of its incidence as the solar. But still they are so radically the same, that what was one, becomes the other. The solar fire, which penetrates the opaque body of the earth, is there dissipated, and becomes elementary; and indeed the whole stock of elementary fire, distributed through the world, depends upon the solar. The elementary, agitated by any violent motion or attrition, and communicated to any proper combustible matter, becomes culinary; and the culinary, when cold and extinct, becomes elementary again.

I have premised these general observations, that we may avoid confusion, by knowing what it is we are inquiring after; and now it appears, that by the name of fire we understand that subtile matter so generally diffused, which, from the sun, is called light; when acting in any sort of fuel, is commonly known as burning fire; and in the pores of bodies, is called æther. As we proceed we shall find farther evidence, such as it would be improper to produce at large in this place,

that