the object glass strongly, the star will appear like a train of wild-fire streaming into the tube. The matter from the candle strikes. upon the rays from the star, and meeting them at various angles disturbs their parallelism so that they enter the glass in many directions; after which the object is lost in the confusion of its own rays.

It hath already been noted by the way, that it is the nature of heat, when in a rising state, to expand or rarefy both solid and fluid bodies: and the reason is this, that, when fire is agitated with that motion which occasions heat, it always acts as if it wanted more room; and this in such a wonderful manner, as if every particle of space in which it exists were a radiant point or centre, from whence it spreads forcibly outwards in every direction: and consequently when fire, thus acting, is admitted into the pores of bodies, their parts must be stretched out, and their dimensions every way increased, according to the degree of fire by which they are acted upon. This expansion by fire is universal upon all bodies, but those whose solid parts are brought nearer together by the evaporation of some fluid which the heat drives out from them and is first to be observed in the

6

fluid

fluid of air, whose bulk, by the heat of boiling water, is increased one third, and with the heat which iron has when it is growing white, hath its elasticity augmented somewhat more than four times, as has been found by experiment. Most of the effects of air are owing to this expanding force of fire, which gives it what we call its elasticity: and though some have reasoned as if elasticity were some property in the parts of air itself, experience shews that this seeming property in the air depends entirely on the fire intermixed with it. When air is united

to

any solid matter in a quiescent state, and is suddenly opened and rendered volatile again by the penetrating power of fire, it is extended to an incredible degree, and its force becomes amazingly great. This is the reason why gunpowder exerts such a wonderful force, the actual ratio of which was investigated by a learned mathematician of the Royal Society, who was happy in his address at calculations grounded upon experiHe found that the air which is

ments*.

volati

* The person here alluded to, was Mr. Benjamin Robins, and the account is to be found at large in his works: but there is an abstract of his doctrine in vol. viii. p. 259 o the Transact. Abr.

volatilized from fired gunpowder occupies about 250 times as much space as when it lies quiescent in the gunpowder: and that the space to which this is again expanded by the heat of the flame, is about four times as much so that the force or pressure of fired gunpowder is about 1000 times greater than the pressure of the atmosphere; which being equal to 14 pounds upon every square inch, the force of gunpowder upon the same must be equal to 14000 pounds; which will account for the astonishing effects of cannon balls.

The particular action of gunpowder is thus accounted for: the sulphur being easily inflamed, as containing in it a large quantity of fire* or phlogiston, is kindled suddenly by the powdered charcoal which is an ingredient of the composition: and the subtile fire of the sulphur, by opening the body of the nitre, sets all its fixed air at liberty, which being extended to great dimensions, and at the same time rendered violently elastic by the heat, explodes with a great noise, and when

+ Εστι δε το θείον ὡς πυρ ανεψυγμενον, εκ ετι θερμον κατ' ενεργειαν αλλα δυνάμει. Sulphur is as it were fire turned

cold, not any longer hot in effect but in capacity." Ne,

mes. chap. v.

when confined exerts a force which far exceeds all the destructive engines in use with the ancients. There is a singular circumstance which attends the firing of gunpow der; that although it seems to generate its own air, and expand by the force of its own materials, a seed of the common thistle with its down, or any other light body, suspended near some fired gunpowder, is always driven inwards toward the powder before it is driven off by the blast. The discovery of gunpowder in the latter ages of the world, about two centuries before those experiments of electricity, which have given us such a new and enlarged prospect of the doctrine of fire, is a fact not to be accounted for but from the consideration of Divine Providence, which opens a way at the proper time to such inventions as may tend either to the instruction or the correction of mankind.

There is another similar composition, in which the power of fire is as great, if not greater, than in gunpowder; it is made of sulphur and nitre as the other, but with the addition of salt of tartar. As the sulphur melts over the fire, its acid is strongly united to the alcaline salt, and the nitre is thereby bound up and confined, till it explodes with

the

the greater violence, by removing the obstruction all at once. This composition cannot be applied to any such purposes as gunpowder, because it is very slow in its operation, and cannot be kindled with a spark. We have an obvious illustration from the effect of gunpowder, that air and fire, both of which are furnished in a wonderful manner by the ingredients, are the great agents in art and nature; the proofs of which are so numerous, that they will be meeting us con- tinually as we proceed, whether the works of art, or those of nature, are the subjects of our consideration.

Water is also expanded with very great force by the action of fire; and it is rarefied to a much greater degree than even air itself. When fire is combined either with air or water, it acts by their interposition upon the surfaces of bodies, which by itself it would easily penetrate, and make its escape insensibly. Water rarefied about 800 or 1000 times, is commonly expanded in the air, and must contribute something to the effects of it; but when water boils over the fire, a vapour rises from it, so powerfully extended by the fire which raises it, that if it is confined, it will force its way through the strongest vessels