along with it, is exposed to the focus of a burning glass in vacuo, the sulphur of the composition will melt and smoke faintly, but will not explode. On the other hand, it is universally known how fire is quickened and increased by a blast of air: on which account it is an established law of nature, that as soon as fire begins to spread itself, a stream of air rushes in from all sides to support it; and the larger the fire, the sharper is the indraught of air. In what manner air acts upon fire, so as to supply it continually with fresh life and vigour, is a question of great importance, and of some difficulty. At present I insist only on the fact, that air is necessary to the continuance of fire, and that the action of fire in general is the joint action of fire and air together. Between these two a double motion is maintained, of fire outwards, and of air inwards. That the matter of the fire goes outwards into the air, is evident from the shadow which any opaque body casts behind it, by intercepting this matter in its course: and the same is clear enough from the heat propagated through the air, and which at a considerable distance from the fire itself, will act as fire, and inflame bodies when it is reflected from a conT 2 cave cave speculum. The continual current of air is manifest to sense in the contrary direction. A silk handkerchief, or any other light body, held near a fire, will be carried into it; and the rushing of the air through all the joints and apertures of the doors and windows of a room, heated by a fire, may be heard and felt very sensibly. It is chiefly owing to the concurrence of the air, that the matter of fuel is wasted and consumed to an heap of ashes. The consumption is very slow, and the fire will live a great while, if embers are raked together, and protected from the forcible impression of the air, by being covered up with their own ashes: this is a common way of preserving fire, to be excited occasionally for domestic uses. Charcoal inclosed in a box of iron, may be ignited for many hours in a strong fire, and when it is taken out will be found to have suffered little or no change. But if the same is set on fire, and the air admitted to it, its surface is very soon covered with a coat of ashes; and as this falls away another arises, till the whole substance is wasted, and nothing remains but a white powder, which is a mixture of incombustible earth and fixed salt. When a diamond is laid in a naked fire, the polish is taken away from the surface; but when it is secured from the air, the fire makes no alteration in it. There is a curious and elegant experiment to illustrate the vehement action of a blast of air upon fire and the parts of fuel. Some metals melt more easily than others; but iron is a metal which cannot be fused without the utmost violence of fire. Notwithstanding all this difficulty, let a bar of iron be laid in a smith's forge, till it has got what they call the white heat: when it is as bright and sparkling as it can be made, let it be taken out of the fire, and let a blast of air from a common pair of bellows be blown strongly against the heated extremity of the iron; which, instead of being cooled by the blast, will become more white and shining than before, till by degrees it rolls about in a liquid form, sending out brilliant sparks in all directions, and falling in frequent drops to the ground. A very considerable part of the rod may thus be liquefied and blown away before the bellows will lose their effect. If a cannon bullet is heated in the like manner, and a large pair of forge-bellows are applied, it affords a glorious spectacle, which can be conceived T3 conceived only by those who have seen it. The eyes are dazzled with the sight, as when we look upon the sun; which, indeed, it resembles more nearly than any other object experiments can exhibit to us. This experiment was shewn several years ago by the ingenious Mr. Henry Horne, a master in the steel manufacture, to Martin Folkes, Esq. when he was President of the Royal Society, and to some other members with him. It is pity some notice was not taken of it in the philosophical collection of that learned body. The smiths, whose business lies at a forge, are so well acquainted with this effect of a blast of air against ignited iron, that they cautiously avoid exposing the metal too near to the nose of their bellows. A fire-man who is but green in his profession, is very apt to be catched with this accident from his bellows, by which he utterly spoils the stuff he works upon by giving it what they call the wind-rot. We proceed now to investigate the principles on which fire is thus supported and invigorated by the concurrence of the air. Some learned men have supposed, that as the air is impregnated with nitrous exhalations, which are of a combustible nature, they they supply the fire with a refined sort of fuel and that fresh air is continually necessary, because that air which has been in contact with the fire is thereby deprived of its nitrous principle, and so rendered effete and unfit for the purpose. But this solution does not correspond with the effect. Nitre, when it is burned, yields a factitious vapour, which has the appearance of air. When a piece of brown paper, dipped in a solution of nitre and then dried, is made to burn in a close vessel, it seems to increase the quantity of air. Therefore, if air feeds the flame of a candle because it is impregnated with nitre, a candle burning under a close vessel should rather increase the quantity of the air; but it is found to lessen it, as we shall see hereafter, There are in fact three different principles on which fire is supported by air, all conspiring to the same end. First, the air, by its pressure, keeps the fire together in a body, and prevents its dissipation. By this effect of the air, the fire is concentrated, and its splendour is very much increased. When the electric spark explodes in air, it is extremely bright and vivid like lightning; but if the same is tried in the exhausted receiver, T4 instead |