was held by chemifts, in the time of Sir Ifaac Newton, to be apyrous, and could not be fufpected, from any of its known qualities, to be of an inflammable nature. Yet this vigilant philofopher did not hesitate to confider it as an un&tuous coagulum, folely from its poffeffing a very high degree of refractive power on the rays of light. For this power he found to depend chiefly, if not wholly, on the fulphureous parts of which bodies are compofed. Late experiments have confirmed this opinion; and fully proved that diamonds confift almoft entirely of pure phlogifton, fince they are capable of being volatilized by heat in clofe veffels, of pervading the moft folid porcelain crucibles, and of being converted into actual flame.

The accuracy of this inference is a striking proof of the importance of judicious and comprehenfive analogies; and of the advantages refulting from the mode of reafoning by induction. For, to use the words of Sir Ifaac Newton,

though the arguing from experiments and "obfervations, by induction, is no demonftration "of general conclufions, yet it is the best way "of arguing, which the nature of things admits "of; and may be looked upon as fo much the

ftronger, by how much the induction is more "general." This improved fpecies of logic was firft recommended and introduced into phyfics, by Lord Verulam, who, at a very early

period of life, faw the futility of Ariftotle's fyllogiftic fyftem, which, proceeding on the fuperficial enumeration of a few particulars, rises at once to the establishment of univerfal propofitions. Dua viæ funt, atque effe poffunt, ad inquirendam et inveniendam veritatem. Altera a fenfu et particularibus advolat ad axiomata maxime generalia, atque ex iis principiis, eorumque immota veritate judicat et invenit axiomata media; atque bac via in ufu eft. Altera à fenfu et particularibus excitat axiomata, afcendendo continenter et gradatim, ut ultimo loco perveniatur ad maximè generalia; quæ via vera eft, fed intentata.*

It is obvious that the force of this inductive method of reasoning must depend on the advancement, which has been made, in the different branches of phyfics. Indeed, it prefuppofes a ftore of particular facts, gradually accumulated, but fufficiently ample, and fit for reduction into their proper claffes. Time and obfervation will be continually diminishing the number, and confequently enlarging the boundaries of thefe claffes, by discovering other relations between them, and pointing out the connection of phanomena, deemed, at firft, diftinct and independent. But it must be remembered that every acceffion to knowledge renews the doubts and difficulties, that refult from ignorance; be

* Bacon. Nov. Organ. Lib. I. Aphor. 19.

cause it presents fresh objects to our inveftigation, and further defiderata to our wifhes. It is this endless progreffion of science, which, by gratifying curiofity with perpetual novelty, and animating ambition with profpects of higher and higher attainments, fometimes gives the attachment to it an afcendancy over every other principle, fo as to render it the ruling paffion of the mind. And as this paffion does not, like the love of virtue, temper its particular exertions, by preferving a proper fubordination in the powers, which it calls forth into action, the wildeft extravagances of emotion and of conduct, have been indulged by thofe, who submit to its uncontrouled dominion. A great philofopher has rushed naked, from the bath, into the streets of a populous city, frantic with joy, on the folution of an interesting problem. But as I have expatiated in another Effay, on the folly of fuch extravagant ardour in the pursuits of knowledge, I fhall close these reflections with the following lines from Milton.

*

Apt the mind, or fancy, is to rove
Unchecked, and of her roving is no end;
'Till warn'd, or by experience taught she learn,
That not to know at large of things remote

From ufe, obfcure and fubtle, but to know

*On Inconfiftency of Expectation in Literary Pursuits; Moral and Literary Differtations, page 183.

ད

That

That which before us lies, in daily life,

Is the prime wifdom; what is more is fume,
Or emptiness, or fond impertinence,

And renders us in things, that most concern,
Unpractis'd, unprepar'd, and ftill to feek.

Par. Loft. Book VIII.

OBSERVATIONS on the INFLUENCE of FIXED AIR on VEGETATION; and on the PROBABLE CAUSE of the DIFFERENCE in the RESULTS of VARIOUS EXPERIMENTS made on that SUBJECT; in a LETTER from Mr. THOMAS HENRY, F. R. S. to THOMAS PERCIVAL, M. D. F. R. S. and S. A. Read May 14, 1784.

DEAR SIR,

IT

T is now many years fince, from some experiments which had made on the effects of you fixed air, applied to the leaves and roots of plants, you, as appeared to me at that time, juftly concluded that fixed air affords a pabulum for plants, which is equal to the fupport of their life and vigour, for a confiderable time. Some of thefe experiments were feemingly contradictory to the refults of thofe related by Dr. Prieftley in his first volume. The doctor therefore requested them in veffels conthat you or I would repeat Z 3 taining

taining pure fixed air. For the doctor had found that plants, confined in pure fixed air, perished fooner than in common air.

I all along understood your meaning to be, not that fixed air, in a pure state, and quite ftagnant, was nutritive to plants; but, that gradually applied, and in a continued ftream, while the plant, at the fame time is not confined from the common air, (in a manner analogous to what may probably take place in nature) plants do receive fuch a portion of nutriment, from the fixed air, as is fufficient for their temporary fupport, even when removed from every other means of receiving their food. This at least was the idea which I always entertained; and the conclufion to be drawn from this theory is, that, probably, fixed air conftitutes a part of the food of plants, when growing in their proper element; fuch air being difcharged by the different manures, which are mixed with their native foil; and this theory, if juft, may lead to confiderable improvements in agriculture. In the third volume of Experiments and Obfervations on different Kinds of Air, Dr. Priestley at the fame time that he acknowledges that "he could con"ceive nothing more fair and decifive than your "experiments," yet declares himself convinced that there must have been fome fallacy in them, and he seems to think that he had detected it in two inftances; the firft fuppofed cause of error

was