4. Gunpowder is alfo fired in all kinds of air, and, in the quantity in which I tried it, did not make any fenfible change in them, except that the common air in which it was fired would not afterwards admit a candle to burn in it. In order to try this experiment I half exhaufted a receiver, and then with a burning glafs fired the gunpowder which had been previously put into it. By this means I could fire a greater quantity of gunpowder in a fmall quantity of air, and avoid the hazard of blowing up and breaking my receiver.

I own that I was rather afraid of firing gunpowder in inflammable air, but there was no reafon for my fear; for it exploded quite freely in this air, leaving it, in all refpects, just as it was before.

In order to make this experiment, and indeed almost all the experiments of firing gunpowder in different kinds of air, I placed the powder upon a convenient ftand within my receiver, and having carefully exhausted it by a pump of Mr. Smeaton's conftruction, I filled the receiver with any kind of air by the apparatus defcribed, p. 19, fig. 14, taking the greateft care that the tubes, &c. which conveyed the air fhould contain little or no common air. In the experiment with inflammable air a

con

confiderable mixture of common air would have been exceedingly hazardous: for, by that affiftance, the inflammable air might have exploded in fuch a manner, as to have been dangerous to the operator. Indeed, I believe I should not have ventured to have made the experiment at all with any other pump befides Mr. Smeaton's.

Sometimes, I filled a glafs veffel with quickfilver, and introduced the air to it, when it was inverted in a bason of quicksilver. By this means I intirely avoided any mixture of common air; but then it was not eafy to convey the gunpowder into it, in the exact quantity that was requifite for my purpose. This, however, was the only method by which I could contrive to fire gunpowder in acid or alkaline air, in which it exploded just as it did in nitrous or fixed air.

I burned a confiderable quantity of gunpowder in an exhausted receiver (for it is well known that it will not explode in it) but the air I got from it was very inconfiderable, and in these circumstances was neceffarily mixed with common air. A candle would not burn in it.

SECTION VIII.

QUERIES, SPECULATIONS, and HINTS.

I begin to be apprehensive left, after being confidered as a dry experimenter, I fhould pafs, with many of my readers, into the oppofite character of a vifionary theorist. A good deal of theory has been interspersed in the course of this work, but, not content with this, I am now entering upon a long section, which contains nothing else.

The conjectures that I have ventured to advance in the body of the work will, I hope, be found to be pretty well fupported by facts; but the present section will, I acknowledge, contain many random thoughts. I have, however, thrown them together by themselves, that readers of lefs imagination, and who care not to advance beyond the regions of plain fact, may, if they please, proceed no farther, that their delicacy be not offended.

In extenuation of my offence, let it, however, be confidered, that theory and experiment

ne,

neceffarily go hand in hand, every process be ing intended to afcertain fome particular hypothefis, which, in fact, is only a conjecture concerning the circumftances or the cause of fome natural operation; confequently that the boldest and most original experimenters are thofe, who, giving free fcope to their imaginations, admit the combination of the most distant ideas; and that though many of these affociations of ideas, will be wild and chimerical, yet that others will have the chance of giving rife to the greatest and moft capital discoveries; fuch as very cautious, timid, fober, and flow-thinking people would never have come at.

Sir Ifaac Newton himself, notwithstanding the great advantage which he derived from a habit of patient thinking, indulged bold and eccentric thoughts, of which his Queries at the end of his book of Optics are a fufficient evidence. And a quick conception of diftant analogies, which is the great key to unlock the fecrets of nature, is by no means incompatible with the fpirit of perfeverance, in investigations calculated to afcertain and pursue thofe analogies.

§ 1. Speculations concerning the CONSTITUENT PRINCIPLES of the different kinds of AIR, and the CONSTITUTION and ORIGIN of the ATMOSPHERE, &C.

All the kinds of air that appear to me to be effentially diftinct from each other are fixed air, acid and alkaline; for these, and another principle, called phlogifton, which I have not been able to exhibit in the form of air, and which has never yet been exhibited by itself in any form, feem to conftitute all the kinds of air that I am acquainted with.

Acid air and phlogiston constitute an air which either extinguishes flame, or is itself inflammable, according, probably, to the quantity of phlogiston combined in it, or the mode of combination. When it extinguishes flame, it is probably only fo much charged with the phlogistic matter, as to take no more from a burning candle, which muft, therefore, neceffarily go out in it. When it is inflammable, it is probably fo much charged with phlogifton, that the heat communicated by a burning candle makes it immédiately separate itself from the other principle with which it was united, in which separation heat is produced, as in other cafes of ignition; the action and reaction, which neceffarily attends the feparation of the constituent princi

ples,