a term denoting the operation of fire upon any inflammable substance, by which it smokes, flames, and is reduced to ashes.

There is not a phenomenon in nature by which the attention of philosophers has been more engaged, nor which has puzzled them more to account for, than this very common operation. To explain it, theories have been invented the most opposite and contradictory to one another that can be imagined; and, till very lately, the state of science did not afford data sufficient to explain it in a rational manner.

By former chemists it was supposed, that the parts of the combustible body itself were converted into fire. Accordingly Sir Isaac Newton proposes it as a query, whether gross bodies and light are not convertible into one another? and many chemists of a more modern date have determined this question in the affirmative, by maintaining that the light of the sun is or contains phlogiston. The interference of the air, however, in most cases of combustion known to us, proved a difficulty in this theory almost, if not totally, unsurmountable; for if the fire proceeds entirely from the combustible body, what occasion is there for any third substance distinct both from the fire and that body to produce combustion? This naturally excited a conjecture, that the fire by which the combustible body is consumed, proceeds in reality from the air, and not from the body itself. And hence we see that Mr Hutchinson's system of fire and air being convertible into one another, might have passed as a rational human theory, if he had not attempted to force it upon mankind as a divine revelation. The modern discoveries in aerology, however, have entirely disproved this hypothesis with regard to our atmosphere considered as a whole, at the same time that they point out the true method, as far as our faculties seem capable of comprehending it, by which this mysterious operation is performed. It is now almost universally known, that the air we breathe is composed of two kinds of elastic fluids, only one of which (called dephlogisticated, pure, empyreal, or vital air) contributes to the support of flame, as well as of animal life; and this part is found to be by far the least in quantity of the atmosphere we breathe. It is computed from good observations, that, among the various component parts of our atmosphere, there is about one-fourth, $\frac{1}{4}$ according to Mr Scheele, or one-fifth according to Mr Cavendish, contained in it; and to this small part alone is owing the combustion of inflammable bodies.

Since the establishment of this important fact, several theories of combustion have been formed. According to M. Lavoisier, dephlogisticated air is a compound of two substances intimately combined; one is called by him the oxygenous principle, and the other specific elementary fire. During the combustion of sulphur, phosphorus, inflammable air, or any other substance of that kind, the oxygenous principle of the dephlogisticated air, according to him, combines with these bodies, to which it has a strong attraction, and forms new compounds of salts and other bodies; at the same time that the elementary fire contained in these is set loose, and becomes sensible, producing heat and flame, according to circumstances. Thus the fire produced in combustion does not proceed from the burned body, but from the decomposition of the dephlogisticated air, in which it is contained in a latent and insensible state; while its oxygenous principle combines with the sulphur, phosphorus, or inflammable air, and forms vitriolic and phosphoric acids, or pure water. In like manner it is also supposed by this theory, 1. That metals are substances absolutely simple. 2. That metallic calces are true compounds formed by the oxygenous part of pure air with the metallic particles; and, 3. That pure water is a similar compound of the same principle with inflammable air.

According to Fourcroy, combustible bodies are those which have a strong attraction to combine or theory. unite with pure or dephlogisticated air; and combustion is nothing else but the act of that combination. This assertion is founded on the following facts: 1. That no substance can be burnt without air; 2. That the purer this air is, the more rapid is the combustion; 3. That in combustion, an absorption or waste of air always takes place; and, 4. That the residuum contains often a very sensible quantity of that pure air which it absorbed, and which may sometimes be extracted from it.

In Mr Scheele's new theory of heat, fire, light, and Scheele's phlogiston, he considers heat and light themselves as compound substances. The former, according to him, consists of phlogiston and empyreal air. The calces of gold, reducible by heat alone, in a retort, show that phlogiston is contained in heat; because it combines with the calces to revive them, and the dephlogisticated air is found in the receiver. The precipitate per se of mercury, if revived in this manner, affords, according to our author, another instance of the truth of his doctrine: "If phlogiston alone (says he) could pass through the retort, there would not be found the empyreal air in the receiver, and the ignoble metals might be revived in the same manner."

Light, according to Mr Scheele, is a compound containing: containing phlogiston and heat, from which both may separate themselves in proper circumstances. A solution of silver in nitrous acid mixed with chalk, and exposed to the sunshine, is revived into a metallic form by the phlogiston of light. Nitrous acid also in a glass vessel, receives phlogiston from light, and becomes of an orange colour; but if the glass be painted black, the acid receives the heat, not the phlogiston. Even the various coloured rays of light, according to our author, contain unequal shares of phlogiston; since the violet rays part more easily with their phlogiston to revive metals than any other. When light is not stopped in its passage, no heat is perceived; but if stopped in its course, the opposing body receives heat, and sometimes phlogiston. Light seems therefore to be the matter of heat, loaded with a superabundant quantity of phlogiston. That which comes out from a furnace, produces heat on the surrounding bodies, which ascends with the rarefied air; proceeds forward in straight lines; and may be reflected from polished surfaces, with this peculiarity, that a concave glass mirror retains the heat, whilst it reflects the light; for although its focus is bright, yet it is not warm. A pane of glass also put before a burning mirror, retains the heat, and allows the light to pass through it.

Fire is the more or less heated, or more or less luminous state of bodies, by which they are resolved into their constituent parts, and entirely destroyed. It requires, that they be previously heated in contact with air; for every combustible body a certain quantity of heat must be communicated, in order to set it in the fiery commotion.

Combustion is the action of heat penetrating the pores of bodies, and destroying their cohesion: in this case the body parts with its phlogiston, provided there be a substance present which has a strong attraction for the inflammable principle. If the heating be performed in open air, the empyreal part, on account of its stronger attraction, unites with the inflammable principle, which is thus set at liberty; from which union the heat is compounded; and scarcely is this heat generated, when the combustible body is still more expanded by it than in the beginning, and its phlogiston more laid open. The more the heat is increased, the more minute are the particles into which the combustible body is dissolved. The empyreal air meets more surfaces; consequently comes in contact with more phlogiston; and, according to its nature, forms an union with a greater quantity of it, which produces a radiant heat. At this moment the constituent parts of the combustible body are so much diffused by the still increasing heat, that the empyreal air, continuing to pour in upon it in streams, attracts the phlogiston in still greater quantities; and hence the most elastic substance, light, is composed; which, according to the quantity of combustible matter, shows various colours.

The last theory we shall here take notice of is that of Dr Crawford.—He has by a great number of experiments endeavoured to show, that bodies which contain a large portion of phlogiston, possess but a small share of specific heat or fire; on the contrary, that those with a great share of this last, contain but little phlogiston; and lastly, those which are deprived of phlogiston, increase their capacity for specific fire. Thus, when regulus of antimony is deprived of its phlogiston, by calcination, which is then called diaphoretic antimony, it nearly triples its specific fire. The same change takes place in crocus maris and in iron. This fact is generally true, whatever be the nature of the substance; and even the aeriform ones are in the same case, for phlogisticated air has very little specific fire, common air has more of it, and deplogisticated air shows a most prodigious quantity. From these facts it is clear, that phlogiston and fire are distinct, and incompatible substances; so that when one enters into the composition of any body, the other of course is expelled from it. Thus metals are calcined in consequence of a double attraction, by which the metal imparts its phlogiston to the air, while the air communicates its fire to the metallic calces; which is further confirmed by the air that is found in metallic calces, whose increased weight by calcination corresponds to the air that is expelled from them by their reduction to a metallic state.

All combustible bodies are absolutely in the same case. By these are meant such bodies as contain a large quantity of phlogiston in their composition, but looely adherent to them. Dephlogisticated air, which is greatly loaded with specific fire, has at the same time a strong attraction for phlogiston; and, in the act of combustion, communicates its fire to the combustible body, whilst the air becomes phlogisticated or loaded with phlogiston. Thus we find, that sulphur contaminates the air, when burned, by the phlogiston it throws into it, and the produced vitriolic acid, if any, becomes impregnated with the same.

In some cases the most intense heat or sensible fire is produced in the combustion; but in others it is very moderate. This variation generally depends on the quantity and quality of the vapours produced during the combustion: when these are very inconsiderable, and the residuum cannot absorb the fire which is emitted by the air, the remainder is precipitated, or diffused all around, and produces a very sensible heat. On the contrary, if the vapours are capable of absorbing it, very little heat is produced. We know, by the most certain experiments, that, for instance, the vapour of water absorbs about 800 degrees of heat beyond that of its boiling state; from whence it follows, that, whenever there is a quantity of watery vapours produced by combustion, very little sensible fire must be felt. So when spirits of wine are fired, the heat then produced by the combustion is very inconsiderable, the greater part being absorbed by the watery vapours that are then produced; but when the phosphorus of Kunkel is set on fire, the heat is very strong; there being but a small quantity of acid to carry off the specific fire that is set loose.

There are the principal theories of combustion that M. Magel have appeared. M. Magellan, from whose notes on Jan's remarks. Cronstedt's Mineralogy the above account is taken, objects to M. Lavosier's opinion, that the oxygenous principle cannot be shown to our senses, nor is it better demonstrated than the phlogiston supposed by the great Stahl and his followers. M. Fourcroy's system he supposes to be less objectionable; but to Scheele he objects from Mr Kirwan, i. That in no instance it appears that phlogiston penetrates glass, much less a compound compound of pure air and phlogiston; and, adly, That if Mr Scheele's notions were true, then other metallic calces, or at least black manganese, would be reduced by heat alone; for this calx dephlogisticates nitrous acid, and has a stronger affinity with phlogiston than it; and therefore ought to decompose the heat with as great facility as the nitrous acid, or even with greater on account of its greater attraction. The former objection M. Magellan does not suppose to be altogether conclusive, as there are many combinations (he says) of two or more substances that pass through bodies, each of which would be stopped before they were combined; and what Mr Scheele has said on light seems to prove that glass is not always quite impervious to phlogiston; but the latter he deems altogether unanswerable.

Having thus rejected three of these theories, he acquiesces in that of Dr Crawford, which, he tells us, "is the most satisfactory concerning the nature and process of combustible bodies and of their combustion, so far as the present state of our knowledge has opened the field of our views into the operations of nature." Before such a full assent, however, is given to any theory, it is altogether necessary that it should be consistent with every known fact, as far as that fact can be investigated by us in our present state of knowledge; and that this is not the case with the theories either of Fourcroy, Scheele, or Crawford, will appear from the following considerations.

1. With regard to that of Fourcroy, it is evidently deficient in one of the essential requisites to produce combustion, even fire itself; for if combustion depends only on the attraction between combustible bodies and pure air, then it ought to take place on all occasions wherever pure air and combustible bodies are presented to each other. But this is not the case; for though we put a piece of unlighted charcoal into a jar full of dephlogisticated air, no combustion will ensue. To produce this it is necessary that the charcoal be already, in part, at least, in a state of combustion, or that fire be applied to it from without. This theory therefore, instead of explaining the matter, gives not the smallest insight into it; since we are perpetually left to seek for the cause of the fire which produced that in question: for the combination of a combustible body with air is the effect of combustion, not the cause.

II. Mr Scheele's theory is so exceedingly contrary to the common notions of mankind, that it can scarce ever be seriously believed. The pure light of the sun can never be supposed by any mortal to consist principally of a substance as gross as the foot of our chimneys, without a degree of evidence of which the subject is quite incapable.

III. Under the article Chemistry, Dr Crawford's theory of heat is fully considered, and found to be insufficient. It is there shown that the degree of specific heat contained in bodies cannot be measured by any method yet known to us; that the phrase, quantity of heat, so frequently made use of by Dr Crawford and others, is vague, inaccurate, and improper; as expressing only the degree of sensible heat extricated, produced, generated, or which becomes perceptible in certain circumstances by us, without regard to the real quantity contained in the body itself, either originally, or after it has parted with that in question. Thus all experiments founded on the quantities of specific heat contained in different bodies must be fallacious and inconclusive. Not to insist, however, on these general arguments, it is contrary to fact, that "bodies which contain a large portion of phlogiston contain but a small share of specific heat," and vice versa, as the Doctor affirms; which will appear from the following considerations.

1. The only methods by which we can measure the quantity of any material substance is either by its bulk or weight.

2. Whatever occupies space, and resists the touch, we have a right to call a material substance, whether we can see it and weigh it or not. Thus air, which is invisible, and not very easily ponderable, is universally allowed to be a substance and not a quality.

3. In cases where we cannot conveniently measure the weight of any substance, its quantity must always be judged of by its bulk. Thus the quantity of air contained in a bladder, or in a bellows, is always judged of by the degree of expansion of either.

4. Heat, which is still more subtle than air, is measured in this way, as Dr Crawford himself acknowledges; for the expansions of mercury are in an arithmetical progression expressive of the real degrees of heat.

5. Applying this rule to bodies in general, we must conclude, that the expansions of all bodies will be in proportion to the degrees of heat which they contain. Thus, if a body is expanded by heat to double its bulk, and in this state remains even when the heating cause is withdrawn, we may then say with justice, that this body contains double the quantity of latent or specific heat that it did before, and so on (A).

6. As the vapour of water is found to absorb a vast quantity of heat, and likewise to become prodigiously expanded in comparison with the water from whence it is produced, we may justly conclude, that the quantity of heat absorbed, or of specific heat contained in the steam, is to the specific heat contained in the water as the bulk of the steam is to that of the water. It is difficult indeed to determine how much steam exceeds in bulk the water from which it is derived: but from some experiments, Dr Black concludes, that it is augmented in bulk between 1600 and 1700 times; and from the great quantity of heat emitted by steam during its condensation, which in some cases exceeds 1000 degrees of Fahrenheit, we have reason to believe

(a) This is not contradictory to the observation that the expansions of all bodies are not in proportion to the degree with which they are heated, nor equal at different times. It is the degree of heat absorbed and entangled among the particles of the body which expands it, not that which flows out from it, and affects our senses or the thermometer. Thus, though a body is heated to 100 degrees, it may absorb only 10; and after it has done so, it may require 300 or 400 degrees more to cause it absorb other ten. believe that the quantity of its expansion is proportionable to that of the heat absorbed.

7. As we thus are ascertained, by the great expansion of aqueous vapour, that it has absorbed a vast quantity of heat, it will evidently follow, that from the expansion of other substances we ought also to know the quantity of heat absorbed by them. To apply this then to the present case. In Dr Priestley's experiments on the conversion of charcoal into inflammable air; he found, that one grain of charcoal, dispersed by the heat of the sun in vacuo, gave six ounce measures of inflammable air. In another experiment, he found that 2½ grains of charcoal gave 15½ ounce measures of the same kind of air. But from a computation of the weight of the air so produced, it appears, that at least an equal quantity of water with that of the charcoal goes to the composition of the aerial fluid. In measuring this expansion, therefore, we may allow one-half for that of the water requisite to form the inflammable air; and hence the grain of charcoal, properly speaking, absorbs only three ounce measures of fire. That this expansion was the effect of fire is very evident: for there was nothing else present but fire, or the concentrated light of the sun; the experiment being performed by means of a burning glass in vacuo. It cannot be a fact then, as Dr Crawford asserts, that a phlogistic body contains but a small quantity of specific heat; for here too small a quantity as one grain of charcoal was made to contain as much specific fire as is equivalent in bulk to three ounce measures. It appears therefore, that the quantity of specific fire contained in bodies is not determined by their being combustible or not, or by their containing phlogiston or not: much less can we believe that heat and phlogiston are incompatible with one another, that where "one enters into the composition of any body the other is of course expelled from it;" since here we find the purest fire we know united in vast quantity with the purest phlogiston we know, and both together constituting one of the most inflammable substances in nature, viz. inflammable air.

8. In like manner must the last part of the Doctor's theory be erroneous, viz. that "in the act of combustion the dephlogisticated air communicates its fire to the combustible body." In the instance just now adduced, the combustible substance, inflammable air, contains already as much fire as it can hold; and according to the general rule in these cases, if it was to absorb more fire, it ought to become still more expanded. But instead of this, when dephlogisticated and inflammable air mixed together in due proportion, are set on fire, they shrink in a manner into nothing; so that it is plain, instead of one communicating its fire to the other, both of them throw out almost all the fire they contain; so that they are no longer air, but water, or some other substance about which philosophers are not yet agreed.

9. Dr Crawford's theory of combustion is liable to the very same objection with that of Fourcroy, viz. that it sets aside the necessity of any external cause to set on fire the combustible bodies. If dephlogisticated air attracts the phlogiston of the combustible body, and the phlogiston in the latter attracts the fire of the dephlogisticated air, the consequence of which is combustion; then, wherever dephlogisticated and inflammable air are mixed, combustion ought immediately to ensue. But this is not the case. A candle, a spark of electricity, or, in a word, some body already in a state of combustion, must be applied before we can produce the effect in question. We must therefore seek for the cause of combustion in the burning body applied, which will be found equally inexplicable; and thus we cannot proceed a single step in real knowledge, though assisted by all that Dr Crawford has advanced.

10. The theory of M. Lavoisier, notwithstanding Magellan's criticism, seems to come much nearer the truth than that of Dr Crawford. With regard to the existence of what Lavoisier calls the oxygenous principle, it is certainly established on as sure grounds as that of any invisible substance can be. Magellan complains, without reason, that it "cannot be shown to our senses." It has not yet indeed been made visible, per se; but it is found to increase the weight of bodies very sensibly. Perhaps, indeed, it may not be an oxygenous or acidifying principle; perhaps it may be water, or some other substance; but still it is something which, by being combined with elementary fire, is expanded into a vast bulk, and which, by being deprived of this fire, shrinks into its former dimensions. Thus it manifests itself to be a real substance; and not only so, but a terrestrial gravitating substance; and which, even when lightened by a mixture of charcoal so as to constitute the solid part of fixed air, has been shown nearly to equal the density of gold. In this respect, therefore, M. Lavoisier's theory is faultless, as well as in that which affirms that in the act of combustion the dephlogisticated air parts with its fire; but it is imperfect in this respect, that he does not consider the quantity of fire contained in the inflammable body, which is thrown out at the same time, nor the occasion there is for some body in a state of actual inflammation to begin the combustion. That the combinations mentioned by him do actually take place is not denied; but they are undoubtedly consequences of the combustion, not causes of it, as they are generally supposed. To understand this subject fully, therefore, it will still be necessary to consider farther.

11. Under the article Chemistry, already quoted, it is shown that heat and cold are not essentially distinct theories, but that heat consists in the motion of a certain subtle and invisible fluid from a centre towards a circumference, and that cold consists in the action of the same fluid from a circumference to a centre. In other words, when elementary fire acts from any body outwards, we say that body is hot, because it heats other bodies; but when it flows from others into any particular body, we call the latter cold, as depriving the neighbouring bodies of part of their relative quantity of heat. We may farther illustrate this by the example of electricity, where the fluid rushing out from any body produces a kind of electricity called positive; but, when entering into it, produces another, opposite in many respects to the former, called negative electricity. In like manner all bodies in the act of throwing out elementary fire are hot, and in the act of absorbing it cold. Vapours of all kinds, therefore, ought to be naturally cold; and experience shows that they really are so; for, by means of evaporation, very intense degrees of cold may be produced. See Cold and Evaporation. 12. In most terrestrial bodies the heat easily flows out from one to another, and therefore they are in a manner indifferent as to the state of being either hot or cold; but in vapours, the heat, having once flowed into them, continues to have a tendency to do so without regard to the external temperature of bodies. Hence these fluids are naturally cold to the touch; and those who have been immersed in clouds on the tops of high mountains or otherwise, have uniformly related that they found the vapour excessively cold; and thus our atmosphere, unless supplied by the powerful influence of the sun-beams, not only becomes extremely cold itself, but likewise cools to an extreme degree the surface of the earth and everything upon it.

13. In all cases therefore, where a quantity of vapour, whether inflammable or not, is collected into one place, there is a constant influx, or at least a constant pressure inwards of the elementary fire existing invisibly all around: which pressure must continue until by some means or other the flow or pressure of ethereal fluid be reversed, and instead of tending from without inwards, is made to tend from within outwards.

14. One method of reversing this influx is by external pressure, or by any other means bringing the particles of vapour nearer to one another. On this subject, a treatise has been written by Dr Webster of Edinburgh, in which he endeavours to establish the doctrine, that condensation is in all cases the cause of heat. That it really is the immediate cause, in a great many cases, is very certain; but it is equally evident that, even in these cases, the cause of condensation must be the ultimate cause of heat. Thus, if a quantity of air be violently compressed in an air-gun, it is found to become hot; but though the compression be the immediate cause, the force by which the compression is occasioned must be the ultimate cause of the heat. The immediate agent, however, by which the heat is produced, is neither the compressing cause nor the condensation, but the efflux of elementary fire from the air, by bringing the particles of the latter nearer to each other. In like manner, when iron is hammered until it becomes hot, the metal may probably be supposed to be condensed, and the elementary fire to be squeezed out of it as water from a sponge; but it is neither the action of the hammer, nor the approximation of the particles to each other, that is the cause of heat; but the flux of elementary fire directed from the iron everywhere from within outwards.

15. Thus we may now at once explain the action of combustion; to do which, we shall take the example of a mixture of inflammable and dephlogisticated air already mentioned. When these are mixed together, there is a constant pressure of the elementary fluid inwards from all quarters into the aerial vapours, by which their elasticity and form as airs are preserved; and this pressure will continue as long as we let them remain undisturbed. But when a burning body is brought into contact with them, the influx of the elementary fire is not only prevented but reversed in that part which comes in contact with the burning body. Thus the whole constitution of both inflammable and dephlogisticated airs is destroyed in a moment; for the subtle fluid, feeling (if we may use the expression) that the pressure is lessened in one place, instantly directs its whole force thither; and the pressure inwards being thus reversed in this part instantly becomes so in every other, and the whole fluid contained in both is discharged with a bright flash and loud explosion.

16. In a similar manner may we explain the combustion of solid bodies. None of these can be ignited without the assistance of external fire. This in the first place rarefies some part of them into vapour; which by means of dephlogisticated air is decomposed in the manner already mentioned; while, by means of the heat thrown out, a fresh quantity of vapour is raised, at the same time that the fire is augmented, and would continue to be so in infinitum, as long as fuel could be supplied. When no more inflammable vapour can be raised, the combustion ceases of course; and the remainder becomes charcoal, ashes, flag, &c., according to its different nature, or the combinations it is capable of assuming with the terrestrial or gravitating part of the pure air by which the fire was supplied.

17. It may now be asked, If the cause of combustion be merely the reversing of the influx of elementary fire, why cannot inflammable vapours be fired in vacuo, by means of heat applied to some part of them externally? Thus, as inflammable air has a constant influx of elementary fire into it, why may not this influx be reversed, and a flame produced, not so violent indeed as with dephlogisticated air, but sufficient to authorize us to say that such a body was actually in the state of combustion? But this, we know, cannot be the case unless some pure air be admitted; for a stream of inflammable air, if nothing else be admitted, will as effectually put out a fire as a stream of water. Here, however, we may reply, "that this would suppose inflammable air to be destroyed by the very power by which it was produced. It seems to be the nature of all vapours to absorb heat without any limitation, as is evident by the increase of elasticity in them by an increase of heat. Elementary fire is one of the component parts of vapour, and no substance can be decomposed merely by the action of one of its component parts. Something heterogeneous must therefore be added, on which one or both of the component parts may act; and then the vapour will be decomposed in vacuo as well as in the open air, though with less obvious circumstances. Thus charcoal once dispersed by heat into inflammable air cannot be decomposed merely by heat, because its tendency is always to absorb this element: But if into a jar full of inflammable air we introduce a quantity of calx of lead, and then heat it, the prelure of the fluid is interrupted in that part where the calx is, and presently becomes reversed by means of the additional heat there, which, at the same time that it furnishes no more charcoal, affords a substance with which the charcoal in the inflammable air may unite. The air is therefore decomposed, though too slowly to produce actual flame." For combustion, therefore, it is necessary that the following circumstances should concur: 1. The mixture of two vapours containing a great quantity of specific fire each. 2. That the terrestrial bases of these vapours should be capable of acting upon one another; but no third substance capable of immediately absorbing the fire should be present. 3. The presence of actual fire in some part, to lessen the prelure of the elementary fluid, set it in motion, and reverse it. This is the case when inflammable and dephlogisticated phlogisticated airs are mixed together. Both these contain specific fire in great quantity. The basis of the one, known to be charcoal, is capable of being united by means of heat to the basis of dephlogisticated air, and of forming with it in some cases fixed air, in others water, or some other substance, according to their various proportions; and after this union is formed, there is no third substance by which the elementary fire may be absorbed. The inflamed body by which they are let on fire first lessens the inward pressure of the elementary fire on one part, by which the bases are allowed to approach nearer each other, and to form a chemical union according to the general observation: But this union cannot be effected without the emission of part of the elementary fire, which being contained in the mixture in great quantity, produces a bright flame. This lessens the pressure still more; a new chemical union and a new flame are produced; and so on as long as any of the materials remain.

When all these circumstances concur, it is not a property peculiar to dephlogisticated air to support flame, though it seems to be so to preserve animal life. It is well known that pyrophorus will burn in common nitrous air, and a candle will burn with an enlarged flame in that kind called dephlogisticated nitrous air. But where any of the concurrent circumstances above mentioned is wanting, no combustion will be produced. Thus, though the steam of water contains a vast quantity of specific fire, and though it is decomposed by passing over red-hot iron, yet no combustion is produced; because, in the very moment of extraction, the elementary fire finds a quantity of phlogiston either in the iron, the water itself, or both, with which it combines, and forms inflammable air, but without any flame.

With regard to the substances which have the property of taking fire spontaneously, as Phosphorus and Pyrophorus, see these articles.