portant fcene opened to his view.' In the following recapi tulation we have collected together the principal deductions which he made from his experiments.

He obferved that plants have not only a power of correcting bad air in 6 or 10 days, by growing in it, as the experiments of Dr. Priestley indicate; but that they perform this important office, in a complete manner, in a few hours; and that this wonderful operation is by no means owing merely to the vege tation of the plant, but to the influence of the light of the fun upon it;

That plants, expofed to the light of the fun, have likewife the furprifing faculty of elaborating the air which they contain, and have, abforbed from the common atmofphere, into real dephlogifeded dir; which they emit, principally from the under furface of their numerous leaves, into the common mafs;-that this operation commences only after the fun has appeared for fome time above the horizon, and is carried on more or lefs brifkly in proportion to the clearness or dulnefs of the day, or the more or lefs favourable expofition of the leaves to the rays of the fun; and that this production of pure air diminishes towards the clofe of the day, and ceafes entirely at fun-fet, except in a few plants which perform this function fomewhat longer than others:

That acrid, ill fcented, and even the moft poisonous plants perform this office in common with the mildeft and the moft falutary; though fome elaborate dephlogisticated air more copiously than others, particularly fome of the aquatic plants:

That, on the contrary, all plants whatever emit a noxious. air, in the night; and even those which excel others in yielding the pureft air in the fun-fhine, furpafs them in the power of infecting the circumambient air in the dark; to fuch a degree that, even in a few hours, they render a large quantity of good air fo noxious, that an animal confined in it lofes its life in a few feconds; and that, even in the day-time, plants shaded by high buildings, or growing under a dark fhade of other plants,

emit an air that is noxious to animals:

That the flowers of plants, univerfally, render the furrounding air highly noxious, equally by night and by day; that their roots, detached from the ground, poffefs the fame property, fome few excepted; but that fruits in general, even the most delicious, have this deleterious quality (though principally in the dark), to fuch an astonishing degree, as to endanger the life of a person who fhould happen to be fhut up in a small and close room, where a great quantity of them were ftored

up:

And lastly, that the light of the fun, fingly, has not the power of purifying any quantity of air expofed to it, without the concurrence of the plants: on the contrary, from one of the Author's

Author's experiments it feemed rather to have contaminated it. We fufpect however that, in this particular cafe, the air expofed to the fun was contaminated by his heat, expelling fome portion of fixed air from the pump water by which it was confined, in the experiment; from which it would be more copioufly extricated, than from the other portion of the fame water that was kept cooler, by being placed in the shade.

This epitome of the Author's principal conclufions is deduced from 125 experiments, circumftantially, though with proper concifenels, related in the fecond part of this work; where they are methodically arranged under different heads. The method generally employed by the Author in making these experiments is the following:

A glass jar is first filled with fresh pump water, which appears to him to be beft adapted to the purpofe; because, generally containing air already, particularly fixed air, it is not fo likely to abforb any part of that emitted by the plants. This jar, thus completely filled, is inverted in a tub of the fame water, which is then expofed to the open air, or rather to the fun-fhine. The plants, or rather their leaves, are then introduced into the jar, through the water. Thus,' fays the Author, the leaves continuing to live*, continue alfo to perform the office they performed out of the water, as far as the water does not obstruct it. The water prevents only new atmospheric air being abforbed by the leaves; but does not prevent that air, which already exifted in the leaves, from ouzing out.'

This air, in fact, does ouze out; for it foon appears upon the furface of the leaves, generally in the form of round bubbles, which increafing in fize fucceffively, rife up to the top of the jar. The air thus collected is found to be true dephlogifticated air; of a greater or lefs degree of purity, according to the nature of the plant which emitted it, bat principally in proportion to the greater or lefs quantity of light to which it had been expofed, and to the time of the expofure. In fome plants, particularly the Nymphaea alba, the bubbles fometimes fucceed each other fo quickly, as to rife from the same spot almost in a continued ftream.

* In confequence of the Author's manner of expreffing homieit in this place, as well as in many other paffages, and from his filence on the fubject, the reader will perhaps find the fame difficulty that occurred to us, in determining whether, in the generality of his experi ments, the leaves and ftalks of plants introduced into the jar were previously separated from the respective plants, or were still connected with them. On making inquiries on this subject, before the Author left the kingdom, we learned that, when the contrary is not expreffed, the different leaves, &c. introduced into the jar were previously feparated from their refpective plants.

There

There are many varieties in this procefs, depending on the peculiar organisation of the leaves in different plants. Some begin very early in the morning to yield dephlogifticated air, and ceafe late in the evening; for instance, potatoe and malva leaves. Others begin the operation very late in the morning, and ceafe very early in the evening; for inftance, the leaves of laurocerafus. The leaves of potatoe plants yield the air bubbles immediately; thofe of malva, in a few feconds; those of the walnut tree, in a few minutes; and the leaves of laurocerafus much later.

The Author infers from his experiments, that the pure or dephlogisticated air, thus obtained from plants exposed to the fan's light, did not antecedently exift in the leaves, in this pure ftate; but is only fecreted out of them, when it has undergone a purification, or a kind of tranfmutation.'-Thus fome leaves of an apple-tree being treated in the manner above defcribed, (excepting their being placed near a fire, inftead of being expofed to the fun); a great deal of air was indeed obtained from them, but it was found to be fo bad as to extinguish flame.

Leaves that had been warmed in the fun's rays, and then haftily plunged into the inverted jar filled with cold water, were found to be remarkably quick in forming air bubbles, and in yielding the best dephlogisticated air. Nor is any dephlogifticated air to be obtained in a warm room, unless the fun shine upon the jar containing the leaves. From hence the Author concludes that the production of this pure air does not depend on the warmth, but chiefly, if not folely, on the light of the fun.

It has been already, in part, obferved, that plants exposed to clear day-light, or fun-fhine, will, in a fhort time, purify air that has been rendered unfit for refpiration; fo as to make it equal to common or atmospheric air in purity. A single leaf of a vine hut up in an ounce phial containing air contaminated by breathing, fo that a candle would not burn in it, restored it to a state of purity equal to that of common air, in the space of an hour and a half.

That the Reader may form fome judgment of the quantity of dephlogifticated air, which the Author obtained from the leaves of plants, treated in the manner above described; we shall select two inftances from the experimental part of this work.

One hundred leaves of the Nafturtium Indicum (which however the Author reprefents as furpaffing the generality of plants in the production of dephlogifticated air, both with refpect to quantity and quality) being put into an inverted jar holding a gallon, and filled with pump water, were exposed to the fun two hours, between ten and twelve. During this time they yielded as much dephlogisticated

dephlogisticated air as would fill a cylindrical jar four inches and an half long, and one inch and three quarters diameter.

The air being taken away, an equal quantity of dephlogisticated air, of equal or ftill greater purity, was collected from the fame leaves during the remainder of the evening, and the next forenoon, without taking them out of the water. This quantity of air greatly furpaffes the bulk of the leaves themfelves; and fhews, fays the Author, to how amazing a quantity the air may amount, that is yielded in a fair day by a lofty tree.'

Twenty-feven fingle leaves of a walnut tree were exposed in the fame manner to the fun-shine, in a fair warm day, from eleven till five o'clock; where they had yielded about one ounce measure of good dephlogisticated air.'

The Reader will likewife naturally wish to know the quality, or the degree of purity, of the dephlogisticated air which the Author obtained in this manner, from the leaves of vegetables. To enable him to form fome estimate on this head, we fhall first observe, that in this air the flame of a wax taper not only burned with such a degree of brightness that it dazzled the Author's eyes; but it excited a crackling hiffing which accompanies the flame when plunged in pure dephlogifticated air.' In fome cafes, fix measures of nitrous air were required, before the faturation of two measures of this air from vegetables could be obtained; and above five-eighths of the bulk of the two airs were destroyed. He had not yet however been fo fortunate as to procure from the leaves of vegetables any air fo pure as that which he obtained from the green matter mentioned in the beginning of this article, as difcovered by Dr. Priestley; two meafures of which, according to the prefent Author, required eight measures of nitrous air to faturate it; and ths of the bulk of the two airs were deftroyed. The pureft dephlogifticated air expelled by fire from red precipitate, being treated in the fame manner, the Author found that ths of both airs had been destroyed when the faturation was completed.

752

Τ

635

It is true that to every one of the experiments here defcribed, there is annexed a number, ftill more accurately denoting the respective degrees of purity of the different airs obtained in each trial. But this manner of afcertaining the quality of the air depends on a peculiar mode of examination invented by the Abbé Fontana; for which we must neceffarily refer our readers to the work itself, where it is particularly defcribed; and the description is accompanied with a plate reprefenting all the inftruments employed in this examination. This method is faid to be fo very accurate, that, in ten fucceffive trials, in which three meafures of nitrous air are fucceffively added to two measures of common air, there was feldom a difference of more than a 500th

part

1

part in the refult, or in the bulk of the air remaining in the tube. The utility of this method will appear the more confpicuous, if it be true, as the Author alleges, that the results of the trials will not be in the leaft degree difturbed or affected by any difference in the ftrength or quality of the nitrous air employed.

The preceding experiments ftrongly confirm the idea, that the action of vegetables upon air, at leaft in the day, is directly oppofite to that of animals. The latter infpire the common atmofpherical air into their lungs, and then emit it from thence, loaded with a foreign, phlogistic, and noxious principle: whereas vegetables, absorbing common air, retain, by fome peculiar œconomy, the phlogiftic matter it naturally contains, for their nourishment or other purposes; and then emit the air, thus rendered pure or dephlogisticated, as being, to them, noxious, or excrementitious. Dr. Priestley's early experiments have shewn that plants thrive in phlogisticated air; and his later trials, as far as he has carried them, feem to prove, on the contrary, that dephlogisticated air is injurious to them.

We have already obferved, that all plants emit a noxious air at night, or in the dark; but its quantity, fays the Author, is fo inconfiderable, in comparison of that of the dephlogifticated air which they yield in the day, that it amounts to very little. On a rough calculation, he eftimates that the poisonous air, yielded during a whole night, does not amount to an hundredth part of the dephlogisticated air which the fame plant would emit during the space of two hours, in a fair day.

The roots, and flowers of plants, however, at least all of the latter kind which the Author could find in his neighbourhood, emit a poisonous air, both by day and by night, though generally in a small quantity. A few flowers of the honeyfuckle (caprifolium) exposed even to the fun's light in the middle of the day, rendered a body of air equal to two pints highly noxious, in three hours. Thefe flowers, fays the Author, like all others, after having thus rendered truly fatal a body of air equal to two pints, have loft nothing of their flavour [fragrance]. The air itself, which they have poisoned, is impregnated with the fame fragrant smell as the flowers themselves :'- -fo that a perfon fhut up in a small and close room, containing a large quantity of the most fragrant flowers, might lofe his life by this moft treacherous of all poifons. The Author has heard of more than one inftance of fudden deaths, which were too probably occafioned by this hitherto unfufpected cause.

Two dozen of young and fmall French beans inclofed, during one night, in an inverted quart jar, containing only common air, rendered it fo poifonous, that a young and lively chicken put into it inftantly fell down motionlefs; and though taken out in lefs than 20 feconds, and immediately put into a

jar