term used by chemists to express that invisible and very much unknown sublimate, which, in conjunction with heat or elementary fire, Phlogiston produces the phenomena of flame or ignition, and gives to metals their splendor; which, in other circumstances, contaminates the white colour of some earths and metallic calces, with black, brown, or other shades; and which in some cases renders the air noxious, and incapable of sustaining the life of animals, or supporting flame, &c. &c.
To give a definition of this principle of inflammability, as the phlogiston is frequently called, has hitherto been found impossible; because, it is so far from being the subject of investigation by itself, that no person has yet been able to procure it by itself; neither is it possible to expel it from any one body, without suffering it, in the very same moment, to combine with another. In the act of burning, for instance, phlogiston is discharged very copiously by any inflammable body; but some part, and that a very considerable one, goes to the composition of the flame. Part of the remainder is carried off by the air, and goes to the formation of foot; another part contaminates the air, and either converts part of the atmospheric air into what is called fixable air, or, according to others, phlogificates it, while the fixable air is separated from the atmosphere itself, of which it is originally a component part; but in all this process, no part of the phlogiston is to be discovered by itself. In like manner, when iron is dissolved in the vitriolic acid, a great quantity of phlogiston is discharged; but in this case also it is altogether invisible, and incapable of being subjected to examination; the only result of this process being a kind of aerial vapour, by Dr Priestley and others called inflammable air; and so, in all other phlogistic processes, though we are assured that the principle is discharged in great quantity, yet it constantly eludes our most diligent search.
Arguments for the identity of elementary fire and phlogiston.
1. Phlogiston is in some cases capable of penetrating the substance of the closest bodies, in such a manner as to be capable of reducing calcined metals to their proper state. 2. The light of the sun appears to contain phlogiston; as it will turn the calx of silver black when exposed to it, even though the calx is included within a glass vessel stoppered in the most careful manner. In like manner, the green colour which the leaves of plants acquire from the solar light is thought to be owing to a communication of phlogiston from it. 3. Dr Priestley has determined, that the electric fluid either is the phlogiston itself, or contains it; because an electric shock will either reduce metals to a calx, or restore them from a calcined to a metallic state.
In considering these arguments, however, it is obvious, that it ought first to be proved beyond dispute, that the substance of fire, light, or electric fluid, can be fixed in bodies in such a manner as to gravitate, and sensibly increase the weight of such bodies. That phlogiston can do this, seems to be demonstrable from a known fact in the conversion of iron into steel; for in this case, the steel produced always exceeds in weight the iron originally employed. But in no case whatever can we combine the matter of fire or light directly with any substance in such a manner, that we can say beyond a doubt, that we have made the flame phlogiston, or the light ponderable. The celebrated Mr Boyle made a vast number of experiments upon this subject, and supposed that he had succeeded, because many bodies, such as lead, become heavier from being exposed to fire being to the action of fire. But later experiments have discovered, that this increase of weight is owing to an adhesion of air, and not of fire or light, as was commonly thought. Neither can it be at all proved, that the fire or light is in these cases converted into air, as some have imagined. Besides, in the only case in which any thing like pure phlogiston is ever separated by itself, at least as far as we yet know, it is so far from appearing like what we would expect from light or heat, that it is the very reverse; and if there was such a principle in nature as positive darkness, the phlogiston would seem to have a much greater affinity to it than to light. The very remarkable experiment in which we now allude is recorded by Dr Priestley (its author) in the following words.
"A very singular decomposition of inflammable air p. 369. I observed in consequence of exposing a great variety of substances to the influence of a sand-heat, which I kept up for several months. Among other things I buried in this hot sand, glass tubes hermetically sealed, composed and previously filled with all the different kinds of air. By heat, I filled them in the following manner.
"Having provided myself with glass tubes about four feet long, and about one third or one half of an inch in diameter, and of such a thickness that I could easily melt them with the flame of a couple of candles and a common blow-pipe, I first sealed the tubes at one end, then filled them with quicksilver, and placed them inverted in a bason of the same. After this, either transferring the air in a bladder from the jars in which they had been standing in water, or generating the air afresh, if it was of a kind not to bear the contact of water, I filled the tubes completely with the kinds of air on which I wished to make the experiment, displacing the quicksilver. This being done, I inclined the tube, and applying the flame of my candles, with some care, (holding the blow-pipe in my mouth only, and keeping firm hold of the tube on each side of the place to which I was applying the heat,) I melted the glass, and took off what lengths of it I pleased; and every piece was of course hermetically sealed. These pieces I marked with a file, keeping an account of the meaning of the marks, that when I took them out of the sand I might presently know with what kind of air they had been filled.
"When I was performing this part of the process with inflammable air in flint-glass tubes, I observed that the places to which I applied the heat were generally tinged black: but I gave little attention to this circumstance, thinking it might be something accidental; and, without any particular expectation, I buried these tubes in the sand together with the others. This was on the 25th of September 1777.
"On the 20th of January following, I examined these tubes, together with every thing else that had been exposed to the same heat. The tube containing the inflammable air was 10 inches long, and by some accident was broke; but it was jet black throughout. At this I was very much surprized, but I did not then suspect that it was at all owing to the inflammable air with which..." Phlogiston, which it had been filled; thinking it might have been occasioned by some phlogistic matter in the sand, or from some of the vessels that had been burnt in the neighborhood. Reflecting, however, on this odd circumstance, and thinking, from the uniformity of the tinge, that possibly it might have been occasioned by the inflammable air, I filled another small glass tube with the same air; and, sealing it hermetically, buried it deep in sand contained in an iron pot, which I set on the fire, and made very hot, nearly red; and taking it out the next day, I found the tube quite black, except a small part on one side of that end which had been uppermost, about two inches higher than the other, and consequently had not been exposed to so great a degree of heat.
"Being now fully satisfied that the blackness of the tube was certainly occasioned by the inflammable air within it, in circumstances in which it could not expand, I proceeded to examine the state of the air. But, in the first place, to assure myself that there had been no communication between that air and the external air by means of some unperceived crack in the glass, I plunged it in water, and, exhausting the air over it, I did not perceive that any bubble escaped. Then, breaking the end of the tube under water, I examined it, and found it not to be inflammable. Sometimes, however, when I have only made the tube just black throughout, by applying the flame of a candle with a blow-pipe to every part of it in succession, the air has still been inflammable.
"Putting two glass tubes, about four inches in length, and a quarter of an inch in diameter, into a sand furnace, I kept them in it two days; when I took them out, and observed, that the tube which I had placed at the bottom of the sand in the greatest degree of heat was nearly melted, and perfectly blue like indigo; while the other tube, which had not been exposed to so great a degree of heat, was of a beautiful jet black throughout.
"Examining the air in these tubes, I found that in the black tube reduced to one third of its bulk, and mere phlogisticated air. It did not make lime-water turbid, was not affected by nitrous air, and was not inflammable. The air in the blue tube, or that which had been exposed to the greatest degree of heat, was reduced to a very small bubble, so that no experiment could be made upon it. I have no doubt, however, that it was phlogisticated.
"At one time I had a suspicion that this blackness communicated to the glass was something precipitated from the iron by the solution of which the inflammable air had been made; but I was soon convinced of the contrary, by finding that the effect was the very same when the inflammable air was made from zinc.
"I soon found that there was no occasion for such a long process to produce this effect, at least upon the glass. For it began to be discoloured the moment it was red-hot, or rather when it became soft; as was evident by holding one of the tubes in an open fire, or in the flame of a candle; for wherever the heat was applied, the blackness took place immediately, without affecting any other part of the tube.
"When I examined this black tinge narrowly, I found that it did not penetrate the glass; but formed a delicate superficial tinge, leaving the glass as perfectly polished as before the process. But the blackness Phlogiston, was indelible: at least it could not be scraped off without tearing the surface of the glass, and it made no change in it with respect to electricity; for the tube thus blackened was as perfect a non-conductor as ever.
"The blue colour of the glass that was most heated, Mr Delaval informed me, was owing to something of iron in the composition of the glass. That it also depended on the degree of heat, I ascertained by placing one of these tubes in a vertical position in the sand-heat. For the lower end of the tube, which had been most heated, acquired a deep blue colour, and it palled into black at the upper end of the tube without any intermediate colour. There was also no other colour higher than the black; so that the first tinge which the glass receives is a perfect black. Yet, viewing the first tinge that it receives by the light of a candle placed beyond it, it seemed to have a shade of red.
"As I was sensible that the blackness was owing to the precipitation of phlogiston from the inflammable air, I thought it possible that some substance which had a near affinity with phlogiston might discharge it; and trying minium, it succeeded immediately. Having filled one of these black tubes with this metallic calx, the moment I made it red hot, the blackness entirely disappeared, and left the tube as transparent as ever it had been.
"In the first experiment of this kind I used minium out of which all its air had been expelled by heat, and which is of a yellow colour. In this process it became whiter, and adhered a little to the glass. When I scraped it off, I could not be quite sure that any part of it had become real lead, but it evidently approached towards a metallic state, by being of a more compact texture than before.
"In this state of the experiments it was suggested by Mr Bewly, that probably the lead in the glass tubes had attracted the phlogiston; and I presently found this to be the case. For when I had filled a green-glass tube with the inflammable air, and sealed it hermetically, as I had done the flint-glass tubes, I exposed it to a melting heat, which is greater than that which flint-glass will bear, without producing any change of colour in it. What remained of air in the tube that did not escape when part of it was melted, was still strongly inflammable.
"It appears, therefore, from this experiment, that the calx of lead, in the form of glass, has a stronger affinity with phlogiston than any thing in the composition of inflammable air, in a degree of heat capable of melting glass. Or, if there be no proper constituent part of inflammable air besides phlogiston, the attraction of the calx is so great as to reduce the phlogiston from an elastic and uncombined state to a fixed and combined one.
"Having by means of these glass tubes effected a complete decomposition of inflammable air, the phlogiston in it having united with the glass of the lead; I thought, that if there had been any acid in its composition, it would then be disengaged, and be found in the tube. In order to find whether there was any acid in it or not, I poured into one of these tubes a small quantity of water made blue with the juice of turnsole; but it came out as blue as it went in." From this curious experiment it appears, that phlogiston tends to make bodies opaque even when transparent before; and indeed something of this kind is always observed in whatever manner we apply phlogiston. The calces of lead or tin, made by dissolution and precipitation with the vitriolic acid, though of the purest white, become immediately tinged with black by an union with the smallest particle of phlogiston; or at least their whitening is so much fulfilled, that the tendency of phlogiston to produce a black colour is visible on all occasions. Metals also, when combined with phlogiston, are exceedingly opaque; but, when deprived of it, and exposed to a very strong fire, they melt into a transparent glass. Now, if phlogiston was the same thing with fire or light itself, we should be apt to suppose that such bodies as had been longest exposed to the action of fire would contain most of it; but this is not found to be the case. Neither can it be supposed that fire in an active state is capable of expelling fire in a quiescent state; because it must be with this element as with water: succeeding quantities of water, if they are violently urged on, will expel the quantity which lies before them; but at last, a certain quantity will remain in the substance through which the water passes; and this quantity will be the same whether the substance has been immersed in a swift or slow running stream. In like manner, when a substance is exposed to the action even of the most violent fire, that element can only pass through it, as we might suppose water to pass through a sieve, while it remains in the fire; but when it is taken out, and suffered to cool, a certain quantity of the fire must necessarily be supposed to become quiescent, because it has nothing to expel it. It would seem probable, therefore, that if phlogiston was of the same nature with fire, no other change could be made upon bodies by exposing them to the action of that element, than melting, or altering their form; but we evidently see that the fire carries off one part of their substance, and leaves another.
Again, with regard to the calcination of metals by the electric fluid, it does not appear to act differently from common fire. An electric spark made to pass between two ends of wires, will deprive them of some part of their phlogiston: but this cannot prove that the electric fluid is the phlogiston of which they are deprived, because in this case they would contain less electric matter than before; but of this we can have no certainty, because all bodies seem to be full of this fluid, and we have no method of measuring the quantity contained in different bodies. As to the reduction of metallic calces by the electric spark, the fact is denied by some eminent foreign philosophers; but although it should be granted, yet we must observe, that a spark of electricity cannot be procured but by making the fluid pass through a small space of air interposed between two ends of wires, or some similar substances. Now though in this case the spark should reduce a metallic calx lying between these two wires, yet this could never prove that the electric fluid itself was phlogiston; because, in passing from one wire and entering the other, a quantity of phlogiston must necessarily be extricated from the wires by the violent impulse of the fluid. We must suspect, therefore, that the electric matter in this case only gives to the calx the phlogiston which it takes from the wires:
It hath been observed, that by making the electric spark pass a great number of times through a small quantity of air, the latter becomes diminished and phlogisticated. But in this case also the phlogiston is evidently derived from the metallic points between which the spark must pass. The argument therefore drawn by Dr Priestley from this fact, for the phlogistic nature of the electric fluid, cannot hold. It is the same thing as calcining metals confined in a certain quantity of air by a common fire; for in this case the air is always found to be phlogisticated.
The arguments drawn from the blackening of the calces of silver by the solar beams, seem also to be inconclusive. Glass, as appears from Dr Priestley's experiment above related, will imbibe phlogiston, and the black will also part with it, without losing its texture as glass. Possibly the case may be the same with the calces of silver just now mentioned. The glass, though it appear to us quite clear and transparent, may contain some phlogistic matter, which by the action of the solar light may be transferred to the calx; or it may come from the air contained in the glass. At any rate, it is impossible, from a single experiment, to deduce such an important consequence, especially as it is attended with a singularity which at once overthrows almost every inference we can draw from it. If we suppose the light of the sun to be phlogiston, much more ought we to suppose the light of a common fire to be the same: and if this were the case, then the lunar calx ought to be blackened by exposing it to the light of a common fire as well as to the rays of the sun. But this is found not to be the case; for though we expose the calx of silver to a common fire ever so long, no blackness will take place. It is the same thing with regard to the green colour of plants; those from for this is produced by the solar light, and by that colour of light only: so that we must own it to be in some respects specifically different from the light of our common fires; but as these differences are totally unknown to us, it is plain, that until our knowledge in this respect is increased, we cannot argue conclusively concerning the nature either of the one or the other.
In Dr Priestley's treatise on air, he supposes that it is the phlogiston which gives elasticity to that fluid. But in this also that eminent philosopher seems to have been mistaken. It is certain that elasticity is not necessarily connected with phlogiston, since it may be entirely deprived of its elasticity, and fixed in the substance of solid bodies; of which Dr Priestley himself gives a most remarkable instance in the experiment above related of the inflammable air contained in the glass tube. It is impossible that the phlogiston, while in a quiescent inactive state, can either be in a state of expansion, or have a tendency to expand; because this would be supposing it to have two natures diametrically opposite to each other at the same moment. If therefore this substance has some qualities peculiar to its state of quiescence, and others peculiar to its state of expansion, we must of necessity suppose that there is some third substance, by means of which its properties are changed at different times. Now we know, that this third substance is the fire; or rather the fire and, and air combined, for fire alone will not expel phlogiston. If then the fire originally gives its elasticity to phlogiston, it is in the highest degree probable, that to this element it owes its permanent elasticity also.
Some authors have been of opinion, that heat is produced by what they call the evolution of phlogiston; and Dr Dugald Leslie, in his treatise on animal-heat, labours throughout a whole chapter to prove, that the evolution of phlogiston is attended with heat. But granting that it is so, if we suppose this evolution to be the cause of heat, we are certainly wrong; for the cause of heat would then be whatever evolves the phlogiston. Dr Leslie supposes that the action of the blood-vessels evolves the phlogiston; and in this case the action of the vessels, not the evolution of phlogiston, is the cause of heat. But in many cases the argument will not hold: for there are numberless instances where phlogiston is evolved in great quantity, without any considerable degree of heat being produced; and in those where the greatest heat is produced, the phlogiston seems not so properly to be evolved, as to be destroyed in such a manner that it can never afterwards be found.
Some philosophers, and among the rest Mr Schele, have supposed, that fire consists in a chemical combination of phlogiston with dephlogisticated air; in which case, says he, the compound becomes so fine and subtle, that it passes through the pores of the most solid bodies, glass itself not excepted. But this supposition is entirely contrary to the analogy of nature. It is certain, that both the air and phlogiston, separately, are incapable of passing through glass; excepting in certain circumstances, where hot glass is found to be penetrable by phlogistic vapors: nor have we any instance of a mixture of two thick liquids producing a thin one. It seems indeed impossible that this could be the case without a change of nature in both fluids; and no substance can change its nature of itself. Certain substances indeed there are, which appear to change their natures by being mixed; but if we consider the matter fairly, we will find that this change is only in appearance. Thus, if powdered cream of tartar is mixed with chalk or quicklime, the mixture appears to deliquesce in the air, which is a property belonging neither to the chalk nor cream of tartar. But here a quantity of alkaline salt is produced, which naturally deliquesces, and occasions the deception. In like manner, when a neutral salt is formed by an acid and alkali, both of which taken separately will deliquesce in the air, yet the nature of these component parts are not changed though the neutral salt does not deliquesce: only their attraction for each other is so strong, that their attraction for water becomes less than that of the air for the same element; and of consequence the salt remains dry. But at any rate, there is no possibility of making a saline solution more fluid than water. Every substance which can be dissolved in water or an aqueous fluid, always takes something from its fluidity; and the same thing happens with oil, or any other fluid thicker than water. It is incredible that two kinds of oil, both of them having a thick consistence when separate, should yet, by simple mixture, become as fluid as water: yet on a similar supposition, which cannot be explained by any known fact in nature, proceeds phlogiston, the whole of Mr Schele's hypothesis.
It may indeed be argued, that phlogiston is by itself capable, in some cases, of pervading solid bodies; can penetrate and consequently may communicate to the common state hot atmosphere, by a chemical combination with it, such glass, and a degree of fluidity, that it will pass through the pores of glass or any other solid substance. In support of this hypothesis it may be urged, that Dr Lewis affects his New Having seen globules of lead revived by the action of man's Che- the phlogistic vapors of charcoal, even in the middle of thick pieces of glass, where there was not the least appearance of a crack. It is certain also, that though we deprive mercury of its phlogiston as much as possible by means of the nitrous acid, it will rise in its native form by distillation in the closest vessels.
But in these, and other similar instances, we are to consider, in the first place, that by heat the pores of all bodies are opened, and consequently made liable to receive substances, which in their natural state they would not receive. All fluid substances also have their fluidity increased by heat. Thus, we will find that warm water will pass through a thick cloth in a much more full and copious stream than an equal quantity of cold water will do. Now, when glass is much heated, its properties are remarkably changed. It loses its transparency, and becomes a conductor of electricity; and if it then conducts this fluid, or modification of a fluid, which at other times it cannot do without being shattered to pieces, this evidently shows that it may also conduct, or be penetrable by, vapors of various kinds. This will account in a satisfactory manner for the revival of lead in the heart of solid pieces of glass; for it is not pretended that a revivification of this kind will take place in the cold, even tho' the glass should be exposed to the greatest quantity of phlogistic vapors that can be imagined. Hence the inference must be very strong, not that heat derives any of its activity from phlogiston, but that all the activity which phlogiston possesses is derived from heat.
Again, with respect to the calcination of mercury, Why the and its revival by mere heat, it must be remembered, calces of that neither calcinations nor revivifications of metals mercury can take place in vacuo. Mercury may indeed be reduced to a kind of calx by exposure to the fire, but out added then it must also be exposed to the air at the same time; and indeed the case is the same with all metals. It is probable, therefore, that all of them receive something from the air, and this is abundantly confirmed by experiments; for no calx can be reduced without an emission of air at the time of reduction. The common metals on this occasion emit fixable air; but the calces of mercury emit dephlogisticated air. If the air is dephlogisticated, it is plain that the calx must be phlogisticated; because the air of our atmosphere is not in a pure dephlogisticated state. The phlogiston it contains must therefore be disposed of in some way or other, and it is in the highest degree probable that it remains with the calx. But a phlogisticated calx becomes a metal, when exposed to a proper heat. In the revival of mercury, therefore, the phlogiston probably comes from the air. Silver and gold, which are indestructible by fire, probably owe this quality to their having little attraction for fixed air, but a great deal for phlogiston. The consequence Phlogiston of this must be, that if a calx of silver or gold is exposed to the air and to the action of heat at the same time, the calx will imbibe the phlogiston rather than fixable air; and of consequence will always appear in a metallic state, though it should be calcined ever so long.
Having thus shewn at some length, that phlogiston cannot with any probability be supposed to have active qualities of its own distinct from those which it receives from the fire, we must next inquire into its nature from the few properties which it is certainly known to possess. These properties may be reduced to the following:
1. It is capable of being reduced to an exceedingly thin, permanently elastic, and light vapour; in which state it has all the properties of a species of air.
2. It may be deprived of this elasticity, and combined with terrestrial bodies; in which case it becomes ponderable, and gives a remarkable degree of opacity to all bodies with which it unites itself.
3. In a great many cases it is a means of producing and preserving flame; especially when combined with terrestrial bodies.
4. In some cases it appears to give a violent explosive power to common air.
5. When combined with air in a certain degree, it deprives it of the very essential properties of supporting flame or animal-life.
From a general consideration of these properties, it appears probable that phlogiston is a terrestrial substance, expandible indeed in the highest degree, but which may also be reduced to a solid, and in which that state of solidity seems to be endowed with no other properties than those of passive terrestrial matter, owing all its activity to a combination either with fire or air.
In considering the phlogiston with regard to its property of sustaining flame, we find that the greater quantity of the inflammable principle there is in any substance to be inflamed, the more easily and fiercely it burns; and the same effect follows, if the air is perfectly free from phlogiston, though the inflammable subject should contain a smaller quantity of it. But if we invert this proportion, loading the air with phlogiston, and at the same time giving a great quantity of it to the fuel, the fire will infallibly be extinguished, though made ever so fierce originally.—Now to understand the reason of this, we must call to mind what hath already been delivered in various parts of this work, concerning the nature of electric fluid, heat, vapour, fire, and flame. Under these articles it has been shown,
1. That fire, heat, light, and electricity, are only so many different modifications of the same fluid, which we shall here call the etherial fluid.
2. That the etherial fluid acting as fire or heat, acts from a centre to the circumference, in every part of the heated body.
3. That the same fluid acting as light, vibrates as from a centre, but that the centre from which it vibrates is at a distance from the body on which the light falls.
4. When a great quantity of light falls upon any body, that is, when the vibrations of the etherial fluid towards it are exceedingly vehement, they penetrate its substance, however solid it may be, and a new vibration takes place within the substance of the body itself. The centre from which the vibrations proceed being then within the body, heat is the certain consequence; and therefore every intense light will in a short time produce heat.
5. In certain bodies the etherial fluid may have a vibration, or tendency to vibration, within the substance of the bodies themselves, though by reason of their texture this vibration is not observed externally. In this case the heat is said to be latent, and the body is cold to the touch, as is the case of vapour.
6. When this tendency to expand becomes exceedingly great, the particles by which the etherial fluid is confined must yield to its impulse, and the body becomes hot to the touch; or if the impulse is very violent, it breaks out into flame, and burns fiercely.
7. Flame cannot exist without a decomposition of vapour; and this decomposition takes place as soon as more latent heat is forced upon the vapour than it can contain.
8. Vapour will absorb heat to a certain degree; and in proportion to this absorption, it cools those bodies which it touches, or extinguishes fire when blown upon it.
9. Vapour, having absorbed as much heat as it can contain, when blown upon a fire, instead of extinguishing it, is instantly decomposed, and is converted into violent flame, returning back all the heat which it had formerly absorbed.
Under the article Flame, it has also been shown, that probably no vapours whatever are absolutely uninflammable, though some are set on fire with much more difficulty than others. Those which are easily inflamed we call inflammable, and those which cannot be inflamed without a great deal of difficulty are called uninflammable. As, therefore, among all this variety of vapours there must be one more inflammable than all the rest, it is very natural, if it is frequently to be met with, to call it the phlogiston, or inflammable principle. Now, we find a certain kind of substance to universally diffused throughout nature, that scarce any thing seems to be perfectly divested of it. This substance is oil, and it is universally allowed to contain a great quantity of the phlogiston. But, when we come to analyse the substance of oil itself, we find it composed of an earthy matter which remains in the retort after distillation; and of a fluid, thinner than the oil originally was, which arises in vapour; but if this thin oil be subjected to a second distillation, it again leaves a quantity of fixed matter, and yields a portion of oil still more fluid than before. By a repetition of the process, more earth is still produced, the oil which arises becomes less and less in quantity, till at last it cannot by any means be collected.—Hence it is probable, that oil is composed of earthy particles disposed in such a manner that they are easily fitted for absorbing heat, and thus being converted into vapour; at the same time that they are also very easily capable of being decomposed, and of parting with the heat, after which they resume their original terrestrial form.
From a consideration of all this, we are naturally led to conclude, that the phlogiston, properly so called, only a portion of empyreumatic oil, that is, of oil which is empyreumatic. which is partly decomposed by having been exposed to a violent heat.—This hypothesis will solve all the phenomena in which phlogiston is concerned.—Alkalies and calcined earths are found to attract phlogiston powerfully, and so likewise they do the groser oils.—The vitriolic acid unites with phlogiston, and so will it do with oils of almost every kind.—The nitrous acid unites more powerfully with phlogiston than most other substances; but the same acid will unite with many kinds of oils so violently, as to set them on fire.—Phlogiston itains every thing black which it touches, and so does empyreumatic oil.—Phlogiston, when combined with air, will extinguish flame; but oil will do so likewise, if it is thrown on a fire in too great quantity. The reason is the same in both cases. Oil absorbs fire in great quantity before it can be raised in vapour; and until it can be raised in vapour, it cannot be kindled. After it is reduced to vapour, it still absorbs more before the vapour is converted into flame. If then the phlogiston is poured upon flame while it is in this absorbing state, the undoubted consequence must be, that instead of adding any thing to its heat, it will take something from it; and if the quantity is sufficiently large, the flame must be extinguished. In like manner, when oil in its liquid state is poured upon fire, it absorbs part of the heat; and if the quantity of oil poured on is capable of absorbing more heat than the fire can give, an extinction of the flame will undoubtedly take place. But if phlogiston is poured upon flame, after having absorbed as much fire as it can contain, it will then, instead of extinguishing, increase the fire to a great degree; because the texture of it is entirely decomposed, and it throws out at once all the fire it had imbibed before. In like manner oil, if heated till just ready to inflame, will burn fiercely if thrown into a fire though in even so large a quantity. Nay, if oil is violently heated in a vessel over the fire, and that vessel afterwards set down upon the cold and moist ground, the oil will be set on fire. The reason of this is, that the superabundant heat contained in the vessel and lower parts of the oil, is, by the cold, violently forced towards the surface, whence a thick vapour is continually arising. This vapour has already as much latent heat as it can contain; but more being continually forced upon it, in attempting to fly from the cold below, the vapour is entirely decomposed, and throws out not only that quantity which the cold forces upon it, but all that the oil had imbibed while it continued over the fire.
Hence we may account for the generation and phenomena of inflammable air. This is produced in the first place by acids, in the second place by the distillation of inflammable substances by a strong fire, and in the third place by the vapour of vitriolic ether.
1. When the vitriolic acid is poured upon filings of iron, a vapour arises, which is not only capable of inflammation by itself, but explodes with violence when mixed with common air, upon the contact of flame. Here we must consider, that the vitriolic acid contains a great deal of latent heat, as is evident from mixing it with water, when the mixture grows very hot, and shrinks in bulk. This will likewise happen when the diluted acid is poured upon iron filings; for then the mixture again grows hot, which shows an extrication of heat formerly latent somewhere. Along Phlogiston, with this heat, the phlogiston of the metal is discharged, and of consequence the latter absorbs as much fire as it conveniently can. The union is here perfectly complete: for as it is made beneath the surface of an aqueous fluid with which the phlogiston has but little tendency to unite, (for oil and water do not readily mix with one another), the inflammable principle absorbs just as much of the fire as it can contain; the superfluous quantity being dissipated in the air. It then remains an invisible, elastic, and very light vapour; because the greatest part of its composition is pure fire, which never could be proved to have any weight; and therefore inflammable air is much lighter than common air, which always has a considerable quantity of water in its composition.—This being the case, it is plain, that when any flaming body is immersed in this vapour, that part of the vapour which touches the flame must be kindled, and the phlogiston, being emptied of the greatest part of the fire it contained, will now combine with the common air, and phlogisticate it; that is, it will combine, loosely with it, and, not having so much fire as it could contain, will greedily attract more from every ignited substance which comes in its way, and will therefore extinguish fire as far as its operation goes.
It remains only now to shew the reason of the explosion of inflammable air when mixed with common air; and this most probably arises from the extreme inflammability of the phlogistic vapour when fully saturated with heat. By this means almost every part of it takes fire at the same instant; and thus a considerable quantity of air being violently heated at the same moment, must expand in proportion, and a considerable explosion take place. When dephlogisticated air is mixed with inflammable air, the explosion is much greater; and the reason is plain, because in common air there is always a quantity of phlogistic vapour not saturated with heat. This vapour absorbs a quantity of the heat thrown out by the decomposed inflammable air; and consequently lessens that which is communicated to the common air, and on which the explosion depends. But in the dephlogisticated air, as little or no vapour of this kind exists, the heat which the inflammable air throws out is almost all communicated to the former, and therefore the explosion is much greater; especially as dephlogisticated air increases the power of any kind of flame to such a degree, that by its means the heat of common fire is made equal to that of a burning mirror.
2. When inflammable air is produced from a distillation of wood or any other kind of inflammable substance, it is emitted at the time that the oil comes over: and therefore we may justly conclude, that it is nothing else than a part of the vapour of that oil which has imbibed so much fire, and is become so intimately combined with it, that it cannot be condensed without an entire decomposition; for this is the case with inflammable vapours when they have got a certain quantity of this fluid in them. Thus, the grose vapour is raised by the fire, but is not thoroughly penetrated by it. The fire indeed expands it till it becomes lighter than the common atmosphere. It is hot to the touch, and therefore its heat is ready to be absorbed by every cold substance which. Phlogiston, which it touches; of consequence the vapour condenses without inflammation: but when the fire has entered into a more close combination with it, so that the expansive force of that element is totally confined by the particles of the oil, the vapour is then cold to the touch, yet full of internal or latent heat, that it is ready to take fire on the slightest contact of a burning body. The vapour is then inflammable and incondensible; and, being perfectly transparent and invisible, is called inflammable air.
3. The production of inflammable air from vitriolic ether is so easily accounted for after what has been said, that little further notice needs be taken of it: only it may be observed, that the vapour of ether itself, even when in its incondensible state, will explode when mixed with common air; which seems to show very clearly, that we are not to look upon inflammable air, when incondensible and perfectly invisible, as any other thing than an exceedingly subtile vapour or thin smoke, and consequently that the phlogiston itself is only the oily particles which constitute that vapour.
We must now inquire into that remarkable property of phlogiston by which it always diminishes air when mixed with it. In order to understand this, it will be necessary to consider by what means the bulk of air is diminished. This, if we except mechanical compression, can only be done by taking away either part of its elasticity, or some of its other component parts. Now, it hath been shown under the article Fluidity, that all fluids contain a great quantity of latent heat; nay, that on this principle their fluidity entirely depends. Of consequence, the air must contain a great quantity of latent heat; and if any substance capable of absorbing this heat is presented to it, the air must be diminished. This is the case with phlogiston; and therefore air is always found to be diminished in consequence of being phlogisticated. But in proportion as the air loses this heat, and the phlogiston imbibes it, it is plain that the latter must become more and more saturated with it, and of consequence approach nearer and nearer to an inflammable state. Hence we may account for the change of nitrous into inflammable air, mentioned by Dr Priestley. But for a more full discussion of all these subjects, see the articles Inflammable Air, Nitrous Air, &c. in the APPENDIX.
Before concluding this article, however, it will be proper to take notice of some objections which may be brought against the theory above laid down.
1. It is impossible, say the objectors, to prove that there is in nature any substance which properly deserves the name of phlogiston, or sole principle of inflammability. We see that bodies are capable of being set on fire; but before we assume the existence of such a principle as phlogiston in the abstract, we ought to be able at least to define it, that so we might be able to prove its existence in different bodies. But, as matters stand at present, it is impossible for us even to know when we have gained our point. How is it possible, for instance, to know whether the phlogiston in spirit of wine be exactly the same with that in sulphur? Or, by what means can we be ascertained that it is the same principle which gives the splendor to metals that causes bodies emit a flame?
To this it may be replied, that philosophers in general have attributed all these phenomena to one cause: therefore every inquirer ought to search among the natural causes, in order to find one which is capable of producing all these phenomena; and if he finds such an one, there is the highest probability that there is no other; for nature doth not multiply causes where one will serve. Hence it is incumbent on those who would assume two principles of inflammability, to prove the existence of them both; but there is not the same obligation upon those who assume the existence only of one to prove that no more exist.
2. Though bodies may be inflamed, and have a predisposition to be inflamed in certain circumstances, we are not therefore bound to suppose that this predisposition consists in the admixture of any material substance with them.
Here we must attend to the consequence of inflammation, which is a separation of the body into matter of two different kinds; one of which remains, while the other is carried off. As soon as this volatile matter is entirely dissipated, the inflammation ceases. The probability therefore is exceedingly strong, that this matter either is the principle of inflammability itself, or contains it.
PHLOGONIÆ, a class of compound, inflammable, and metallic fossils, found in small masses of determinately angular figures; comprehending the pyricubia, pyroctonogonia, and pyripolygonia.