an invisible elastic fluid, of somewhat greater specific gravity than that of the common atmosphere, and capable of supporting animal life and flame for a much longer time than the air we commonly breathe.

This fluid was first discovered by Dr Priestley, and a very short time after by Mr Scheele, who appears to have been entirely unacquainted with what the Doctor had done. The methods of making it artificially are enumerated under the article Aerology; here we shall make some observations on the way in which nature seems to accomplish the same end. Under that article, as well as that of Fixed Air, and others connected with them, it has been observed, that this kind of air becomes convertible into fixed air by the addition of a certain proportion of phlogiston or charcoal; and that these two ingredients may be again separated by certain means; the dephlogisticated air assuming its proper state, and the charcoal its own native form. A great number of experiments also are there related concerning the effects of vegetables in purifying tainted air, and their emission of the dephlogisticated kind; which has been thought to indicate, that vegetation is probably one of the methods by which nature supplies the great quantity of this fluid necessary for the purposes of animal life. This was suggested by an experiment of Dr Priestley, who had discovered that pump-water, on being exposed to the light of the sun, soon began to emit pure air; and that, after some days, a quantity of green matter accumulated on the sides of the vessel. This was naturally supposed to be of the vegetable kind, till Sir Benjamin Thomson found it to consist of a number of very minute animals. Upon this discovery he disputed the commonly received opinion, that vegetation is employed by nature as a purifier of the atmospherical air. He likewise opposed the conclusion drawn from Dr Ingenhousz's experiments, who had observed that fresh leaves of plants immersed in water, and exposed to the light, emitted a vast quantity of dephlogisticated air. Sir Benjamin justly observed, that a leaf certainly ceased to vegetate after being separated from the plant on which it grew; and therefore the emission of this kind of air could by no means be ascribed to vegetation: as a more decisive proof of which, he deploge likewise urged, that leaves, after being entirely dead and withered, would notwithstanding emit deplogisticated air for a very considerable time. Thus matters seemed to be rendered doubtful; but by an attentive observation of all circumstances relating to these experiments, we shall find that considerable light will be thrown upon the subject.

In the first place, we are to consider that the light of the sun, or at least a very strong light of some kind, is necessary in all these productions of deplogisticated air; for if the apparatus is set in a very dark place, little will be formed, and that of a bad quality. So ignorant are we of the nature of this mysterious fluid, that scarce any inquiry has been made into its mode of operation in producing deplogisticated air. By some the element of light has been supposed to be phlogiston itself, or some modification of it; but since the discovery of the identity of phlogiston and charcoal, it is probable that this opinion will not be held by many. One experiment, however, commonly brought in favour of this supposition, deserves particular attention. This is the property which light has of giving a black colour to the calces of silver; a phenomenon too remarkable, that it has been imagined an indubitable proof of the identity of light and phlogiston. In like manner, the pure and deplogisticated spirit of nitre is found to be converted into the phlogisticated and smoking kind by exposure to the solar rays. The extensive diffusion of the substance called phlogiston, however, is now so well known, that we may reasonably conclude, that in these experiments it was contained in the ingredients themselves, and not in the light. Thus when silver is precipitated by chalk, and the mass turns black on exposure to the sun's rays, we are very sure that the chalk contains phlogiston; and that there is an attraction between the metallic calx and this principle; but we are by no means ascertained of the nature of light, as not being in any manner of way the subject of our investigation, except by observing its effects. As in all other cases, therefore, where light is concerned, we can only say that such a thing is the effect of the operation of light, and not the substance of the element made visible, or converted into some other thing; we have no reason, in this case, to say, that the blackening of the metallic calx is any other than an effect of the light's operation, and not the detention of any part of its substance. This operation may be easily conceived to be the promoting of the union of the phlogiston and calx, which we know that light has a great tendency to do at any rate when we augment its action in such a manner as to make it become fire. The tendency of light, therefore, to promote an union between phlogiston and other substances, will explain this and many other experiments in a very easy manner. In the case of chalk and calx of silver, the action of the light enables the calx to attract the phlogiston of the chalk, and thus become black. If the calx is not precipitated by chalk, but by an alkali, it must be in contact either with air, water, or some other fluid. All these undoubtedly contain phlogiston. With regard to air, it has not been denied that it is a general receptacle of all the decayed and volatile parts of vegetables and animals; that it contains vast quantities of inflammable air, which are constantly emitted from... from various parts of the earth; and, according to the Philogitians, that fluid called phlogificated air, which constitutes the other part of the fluid we breathe, likewise contains a quantity of the same matter. We are not to be surprized therefore that, in the case of the calx of silver, some of these substances should be obliged to part with a little of their phlogiston, sufficient to blacken the metallic earth. In order to prove that light and phlogiston are the same, the experiment ought to succeed in a perfect vacuum, of which there is no probability; though indeed it has never been tried that we know of. In like manner, when spirit of nitre is rendered high coloured and smoking by exposure to the sun in a glass, the phlogiston may come either from the glass itself, which is now found to contain phlogiston, or from the quantity of air which is necessary to be left in the glass, in order to make the experiment succeed.

Thus we may reasonably suppose the blackening of the calx of silver, and the rendering clear spirit of nitre high-coloured, to proceed only from the separation of phlogiston by means of light, and its consequent attraction by the calx or acid; and in other cases, where any similar effect is observed by the exposure of bodies to that element, we are to suppose that it is occasioned by the detachment of phlogiston from one substance and its attachment to another. In the case of the emission of dephlogisticated air by means of light, therefore, we may, by reasoning analogically, conclude, that it is occasioned by the absorption of phlogiston by the substance which is said to emit the air.

Before this matter, however, can be determined exactly, we ought to be well acquainted with the composition of dephlogisticated air itself; and indeed, without this, it seems almost in vain to speak upon the subject. But, notwithstanding the labours and ingenuity of modern aerologists, this point has not been in any degree settled. On examination, it is found to consist of an invisible fluid which does not appear to gravitate, and which in all probability is no other than elementary fire; and of another substance equally invisible, but capable of attaching itself to certain bodies, particularly iron, and adding to its weight very considerably. On attempting to procure this substance by itself, we find the attachment so strong, that no force of fire can separate them. In attempting a decomposition by means of the electric spark, all that has been yet done, even with the greatest power of electricity excited by Van Marum's new machine, is to make it lose some part of its bulk, the remainder appearing by the eudiometer to have undergone no change. Dr Priestley, in his fifth volume of Experiments on Air, acquiesces in the opinion suggested to him by Mr Watt, that the air in question is nothing else but one of the component parts of water united with the element of heat. Allowing this to be just, and indeed there is no experiment hitherto published by which it can be contradicted, the natural method of the production of dephlogisticated air from water may be easily explained. This only requires us to suppose, that the substances immersed in the water, are by the action of the sun's light made to attract part of the phlogiston of the water; in consequence of which the dephlogisticated part of that element, instantly being volatilized by the light and heat of the sun (for heat is necessary for the production as well as light), assumes the properties of dephlogisticated air. But why, it may be asked, does water of itself emit dephlogisticated air without any substance whatever immersed in it? or, after certain substances have been immersed in it, by which this emission is promoted, why does the production of air stop with regard to any particular substance, and any determined quantity of water? Thus; it may be said, if any quantity of water, suppose a pint, yields one inch of cubic air by the immersion of a certain substance into it, why does not this substance attract from it all the phlogiston it is capable of absorbing? Instead of this, the water appears, by the colour it acquires, to be more dephlogisticated than before; and the substance immersed, by being put into fresh water, will immediately occasion the emission of new quantities of air, and this for several times running. But to this it may be replied, that though the substance immersed attracts the phlogiston of the water, the latter will part with it only to a certain degree; and it is well known, that when two substances are united to the point of saturation by chemical attraction, the abstraction of a part of one of them will increase the attraction betwixt the remainder to a great degree. Though the substance immersed in the fluid, therefore, has originally a greater attraction for the phlogiston of the water than the dephlogisticated part of the element itself; yet as the one gradually augments, and the other diminishes, a balance soon takes place. With regard to the green colour generally assumed by the water after such experiments, it most probably proceeds from an accumulation of some terrestrial particles partly supplied by the immersed substance, or perhaps from a disposition to generate the green matter observed by Dr Priestley. That the substance immersed in the water does really part with some particles, is evident, because the water smells of it, as when raw silk is made use of; nor can we suppose that any vegetable or animal substance, such as are found to be alone fit for these experiments, can endure a long maceration in water without parting with a considerable quantity of their component parts. Indeed, under the article Charcoal, it is shown that this body, though now allowed to be pure phlogiston, or the next thing to it, has the power of separating other phlogistic matter, probably such as is more impure, from different substances. Hence its property of whitening tartar, purifying malt spirits, &c. It is not unreasonable, therefore, to suppose, that a dead leaf, though a very phlogistic substance, may have the power of attracting more phlogiston from the water, and thus allowing part of that element to be changed into dephlogisticated air; while, in lieu of the phlogiston attracted from the water, it diffuses a certain portion of its own substance through the fluid, and thus gives it the colour in question. With regard to the other difficulty, viz., that water, when exposed to the solar rays, will emit dephlogisticated air without any thing immersed in it, it may be accounted for from the ease with which the dephlogisticated part of the water is volatilized; so that the attraction of the other is not able to detain it. This we find exemplified in several chemical experiments, as when a volatile alkali is joined with any of the more fixed acids; for in these cases the alkali, notwithstanding the mutual attraction betwixt it and the acid, will be made to fly off by a strong heat.

On the supposition that deploglificated air is composed of elementary fire and one of the constituent parts of water, there is very little difficulty in accounting for the origin of the immense quantity necessary to supply the animal creation with it. Under the article DAMPS, it has been shown that a vast quantity of fixed air is continually elaborated in the bowels of the earth. This is composed entirely of deploglificated air, compounded with a certain portion of phlogiston. Part of this fixed air must be undoubtedly evaporating constantly from the surface, and would certainly infect the lower parts of the atmosphere, were there not some natural causes for its decomposition. One of these undoubtedly is the absorption of the phlogistic part by vegetables, which under the article AGRICULTURE is known to be their proper food. But when the phlogiston is absorbed, a great quantity of deploglificated air is prepared, which supplies the deficiency occasioned by the respiration of animals. It must likewise be observed, that after the deploglificated air has been spoiled either by respiration or the support of flame, it is not for that reason entirely destroyed, but only converted into fixed air, and consequently may again be purified as before.

It may be likewise reasonably supposed, that in cases where vegetation does not take place, and in spring before the plants begin to exert their vegetative powers, and in autumn, or the beginning of winter, when they decay, the vegetable soil itself may perform this office; and indeed the circumstance of fresh vegetable mold affording an agreeable smell when turned up, as is mentioned under the article AGRICULTURE, seems to be a considerable confirmation of this.

The property which water has of absorbing fixed air, and also phlogiston, may likewise induce us to suppose that it acts as a purifier not only of the common atmosphere, but of that which issues from the earth; and hence some waters, particularly that of the ocean, are found to contain air of a purer kind than that of the atmosphere.

As light, however, is an indispensable requisite in all these cases, a difficulty still remains concerning the production of deploglificated air in winter, when the light and heat are so much diminished. In this season, indeed, it is probable that a quantity will be produced greatly inferior to that which the summer affords; but here we may very reasonably have recourse to the immense magazine of the atmosphere itself; which, from the mere circumstance of quantity, must be much more than sufficient to answer the expenses of one season; especially when we consider, that in summer a superabundant quantity is certainly produced. Add to this, that in extreme cold, fixed air seems not only to be less noxious than at other times, but even necessary for the preservation of health*. We must likewise consider, that during the winter season there will be a constant flux of the cold air of the northern and southern regions towards the equator; and this, however imperceptible to the inhabitants, will keep up a constant circulation of atmospherical fluid, so that there cannot be any stagnation even in the calmest weather and most severe and long continued frosts.

Soon after Dr Priestley began his experiments on air, he found that the red colour of the blood depends on the air; that by coming in contact with it, the sanguineous blood became in a very short time of a beautiful florid colour; and that even the under side of a clot of blood, by exposure to the air, will lose its disagreeable dark colour, and assume the same with that of blood newly drawn. These effects are entirely to be attributed to the deploglificated part; and his theory of respiration being a phlogistic process, seems to be fully confirmed by the discovery of the composition of fixed air. For, as fixed air is known to be composed of phlogiston and deploglificated air, we cannot suppose this phlogiston to be derived from any other source than the lungs. The doctor in this theory likewise obviates an objection that might naturally occur, that the air has not immediate access to the blood in the lungs, because of the thin membranes of the vessels which intervene. But, from his experiments, it appears that this is no obstacle. The serum of blood he finds to be capable of transmitting deploglificated air, or at least of propagating this effect through a very considerable thickness of its own substance, as well as through bladders moistened with it; a property which does not belong to any other animal fluid.

As deploglificated air is now known to be the immediate support of animal life, it has naturally been supposed that it might answer valuable purposes in medicine; but the difficulty of procuring it in sufficient quantity has hitherto prevented these hopes from being realized, excepting only in cases of drowning, where it is said to produce very great effects. With regard to any method of preparing it, no farther discovery has been made than what is suggested under the article AEREOLOGY.