Lavoisier's explanation of combustion extended to respiration as well--a process no less important to humans if much less dramatic. It also explained the relationship between metals and earths (or oxides as we now call them). According to the phlogiston theory, earths were simple bodies, and metals were obtained from them by addition of phlogiston; in fact, metals are the simple bodies, and earths are obtained from them by reaction with oxygen. Thus, Lavoisier's work on combustion provided a better understanding of two of the elements of the ancients. And yet Lavoisier did not completely discard the concept of a material associated with fire: we can see some of the evolution of the concept "matter of fire" from phlogiston to caloric in this article. (See Giunta 2001.)
As dangerous as is the desire to systematize in the physical sciences, it is, nevertheless, to be feared that in storing without order a great multiplicity of experiments we obscure the science rather than clarify it, render it difficult of access to those desirous of entering upon it, and finally, obtain at the price of long and tiresome work only disorder and confusion. Facts, observations, experiments--these are the materials of a great edifice, but in assembling them we must combine them into classes, distinguish which belongs to which order and to which part of the whole each pertains.
Systems in physical science, considered from this point of view, are no more than appropriate instruments to aid the weakness of our organs: they are, properly speaking, approximate methods which put us on the path to the solution of the problem; these are the hypotheses which, successively modified, corrected, and changed in proportion as they are found false, should lead us infallibly one day, by a process of exclusion, to the knowledge of the true laws of nature.
Encouraged by these reflections, I venture to propose to the Academy today a new theory of combustion, or rather, to speak with the reserve which I customarily impose upon myself, a hypothesis by the aid of which we may explain in a very satisfactory manner all the phenomena of combustion and of calcination, and in part even the phenomena which accompany the respiration of animals. I have already laid out the initial foundations of this hypothesis on pages 279 and 280 of the first volume of my "Opuscules physiques et chimiques," but I acknowledge that, having little confidence in my own ability, I did not then dare to put forward an opinion which might appear peculiar and was directly contrary to the theory of Stahl and to those of many celebrated men who have followed him.
While some of the reasons which held me back perhaps remain today, facts which appear to me to be favorable to my ideas have increased in number since and have strengthened me in my opinion. These facts, without being perhaps too strong, have made me more confident, and I believe that the proof or at least the probability is sufficient so that even those who are not of my opinion will not be able to blame me for having written.
We observe in the combustion of bodies generally four recurring phenomena which would appear to be invariable laws of nature; while these phenomena are implied in other memoirs which I have presented, I must recall them here in a few words.
First Phenomenon. In all combustions the matter of fire or light is evolved.
Second Phenomenon. Materials may not burn except in a very few kinds of air, or rather, combustion may take place in only a single variety of air: that which Mr. Priestley has named dephlogisticated air and which I name here pure air. Not only do those bodies which we call combustible not burn either in vacuum or in any other species of air, but on the contrary, they are extinguished just as rapidly as if they had been plunged into water or any other liquid.
Third Phenomenon. In all combustion, pure air in which the combustion takes place is destroyed or decomposed and the burning body increases in weight exactly in proportion to the quantity of air destroyed or decomposed.
Fourth Phenomenon. In all combustion the body of which is burned changes into acid by the addition of the substance which increases its weight. Thus, for example, if sulfur is burned under a bell, the product of the combustion is vitriolic acid; if phosphorus be burned, the product of the combustion is phosphoric acid; if a carbonaceous substance be burned, the product of the combustion is fixed air, formerly called the acid of chalk, etc. 
The calcination of metals follows precisely the same laws, and it is with very good reason that Mr. Macquer considers the process as a slow combustion. Thus (1) in all metallic calcinations the matter of fire is evolved; (2) genuine calcination may take place only in pure air; (3) air combines with the calcined body, but with this difference, that instead of forming an acid with it, a particular combination results which is known by the name of metallic calx.
This is not the place to show the analogy which exists between the respiration of animals, combustion, and calcination. I will return to it in the sequel to this memoir.
These different phenomena of the calcination of metals and of combustion are explained in a very nice manner by the hypothesis of Stahl, but it is necessary to suppose with Stahl that the material of fire, of phlogiston, is fixed in metals, in sulfur, and in all bodies which are regarded as combustible. Now if we demand of the partisans of the doctrine of Stahl that they prove the existence of the matter of fire in combustible bodies, they necessarily fall into a vicious circle and are obliged to reply that combustible bodies contain the matter of fire because they burn and that they burn because they contain the matter of fire. Now it is easy to see that in the last analysis this is explaining combustion by combustion.
The existence of the matter of fire, of phlogiston in metals, sulfur, etc., is then actually nothing but a hypothesis, a supposition which, once admitted, explains, it is true, some of the phenomena of calcination and combustion; but if I am able to show that these phenomena may be explained in just as natural a manner by an opposing hypothesis, that is to say without supposing that the matter of fire or phlogiston exists in combustible materials, the system of Stahl will be found to be shaken to its foundations.
Undoubtedly it will not be amiss to ask first what is meant by the matter of fire. I reply with Franklin, Boerhaave, and some of the philosophers of antiquity that the matter of fire or of light is a very subtle, very elastic fluid which surrounds all parts of the planet which we inhabit, which penetrates bodies composed of it with greater or less ease, and which tends when free to be in equilibrium in everything.
I will add, borrowing the language of chemistry, that this fluid is the dissolvent of a large number of bodies; that it combines with them in the same manner as water combines with salt and as acids combine with metals; and that the bodies thus combined and dissolved by the igneous fluid lose in part the properties which they had before the combination and acquire new ones which make them more like the matter of fire.
Thus, as I showed in a memoir deposited with the secretary of this Academy, all aeriform liquids, all species of air are the result of the combination of any substance whatsoever, solid or liquid, with the matter of fire or light. It is to this combination that aeriform liquids owe their elasticity, their specific lightness, their rarity, and all other properties which make them like the igneous fluid.
Pure air, according to this, that which Mr. Priestley calls dephlogisticated air, is an igneous combination in which the matter of fire or of light enters as a dissolvent and in which another substance enters as a base. Now if in any dissolution whatsoever we present to the base a substance with which it has more affinity, it unites instantly and the dissolvent which it has left becomes free; it regains all its properties and escapes with the characteristics by which it is known, that is to say, with flame, heat, and light.
To clarify whatever may be obscure about this theory let us apply it to several examples. When a metal is calcined in pure air the base of the air, which has less affinity with its dissolvent than with the metal, unites with the latter as soon as it is melted and converts it into a metallic calx. This combination of the base of the air with the metal is shown, (1) by the increase in weight which the latter undergoes during calcination, (2) by the nearly complete destruction of the air beneath the bell. But if the base of the air were dissolved by the matter of fire, in proportion as this base combines with the metal the matter of fire should become free and should produce, in evolving, flame and light. It is concluded that the more rapid the calcination of the metal, that is to say, the more of the base of the air is fixed in a given time, the more matter of fire will be freed at the same time and consequently the more noticeable will be the combustion.
These phenomena, which are extremely slow and difficult to perceive during the calcination of metals, are almost instantaneous in the combustion of sulfur and phosphorus. I have shown by experiments, against which it appears to me difficult to make any reasonable objection, that in these two combustions air, or rather the base of the air, was absorbed; that it combined with the sulfur and with the phosphorus to form vitriolic and phosphoric acids. However, the base of the air may not pass into a new combination without leaving its dissolvent free, and this dissolvent, which is the matter of fire itself, should evolve with light and flame.
Carbon and all carbonaceous materials have the same effect on the base of the air: they appropriate it for themselves and form with it by combustion an acid sui generis known under the name of fixed air or acid of chalk. The solvent of the base of the air, the material of fire, is then evolved in this operation, but in less quantity than in the combustion of sulfur and phosphorus because a portion of it combines with the mephitic acid to render it into the vaporous and elastic state in which we find it.
I will observe here, in passing, that the combustion of charcoal under a bell inverted in mercury does not occasion a very great diminution in the volume of the air even when pure air is used in the experiment, for the reason that the mephitic acid which is formed remains in an aeriform state, in contrast to vitriolic and phosphoric acids, which condense into a concrete form as they are produced.
I might apply the same theory successively to all combustions, but as I shall have frequent occasion to return to this subject I will let these general examples suffice for the moment. Thus, to continue, the air is composed, according to me, of the matter of fire as dissolvent combined with a substance which serves it as a base and in some manner neutralizes it. Whenever a substance toward which it has more affinity is presented to this base, it quits its dissolvent, and then the matter of fire regains its properties and reappears before our eyes with heat, flame, and light.
Pure air, the dephlogisticated air of Mr. Priestley, is then, from this point of view, the true combustible body and perhaps the only one in nature, and we see that there is no longer need, in explaining the phenomena of combustion, of supposing that there exists an immense quantity of fixed fire in all bodies which we call combustible, that on the contrary it is very probable that little of this fire exists in metals, sulfur, and phosphorus and in the majority of very solid, heavy, and compact bodies; and perhaps even that only the matter of free fire exists in these substances by virtue of the property which this matter has of coming into equilibrium with neighboring bodies.
Another striking reflection which supports the foregoing is that nearly all bodies may exist in three different states, namely, in the solid, the liquid, which is to say, melted state, or the state of air and vapor. These three states depend only on the greater or lesser quantity of the matter of fire with which these bodies are penetrated and with which they are combined. Fluidity, vaporization, and elasticity characterize the presence of fire in great abundance; solidity, compactness, on the contrary, evidence its absence. As much, then, as it is proved that aeriform substances and the air itself contain a large quantity of fire, so much is it probable that solid bodies contain little.
I would be overstepping the limits which I have prescribed and which the circumstances demand were I to undertake to show how this theory throws light on all the great phenomena of nature. However, I cannot omit remarking upon the ease with which it explains why the air is an elastic and rare fluid. Indeed, fire being the most subtle, elastic, and rare of all fluids, it should communicate a part of its properties to the substances with which it unites, and, as solutions of salts always partake of some of the properties of water, so dissolutions by fire should retain some igneous properties.
It will be seen, then, why we cannot have combustion either in a vacuum or in any aeriform combination where the matter of fire has a very great affinity with the base with which it is combined.
We are no longer obliged, following these principles, to admit the presence of a large quantity of the matter of fixed and combined fire even in the diamond itself and in a great number of substances which have no quality like that of the matter of fire or which possess properties incompatible with it. Finally, we are not at all obliged to maintain, as did Stahl, that bodies which increase in weight lose a part of their substance.
I remarked above that the theory proposed in this memoir could be applied to the explanation of a part of the phenomena of respiration, and with this I will finish.
I showed in the memoir which I read at the public meeting of last Easter that pure air, after having entered the lungs, leaves in part as fixed air, or the acid of chalk. Pure air, in passing through the lungs, undergoes then a decomposition analogous to that which takes place in the combustion of charcoal. Now in the combustion of charcoal the matter of fire is evolved, whence the matter of fire should likewise be evolved in the lungs in the interval between inhalation and exhalation, and it is this matter of fire without doubt which, distributed with the blood throughout the animal economy, maintains a constant heat of about 321/2 degrees Réaumur. The idea will appear to be hazarded at first glance, but before it be rejected or condemned I beg you to consider that it is founded on two certain and incontestable facts, namely on the decomposition of the air in the lungs and on the evolution of the matter of fire which accompanies all decompositions of pure air, that is to say, all changes of pure air to the state of fixed air. But that which further confirms that the heat of animals stems from the decomposition of the air in the lungs is that only those animals in nature which respire habitually are warm-blooded and that their warmth is the greater as respiration is more frequent; that is to say, that there is a constant relation between the warmth of an animal and the quantity of air entering, or at least converted into fixed air in, its lungs.
Furthermore, I repeat, in attacking here Stahl's doctrine my object is not to substitute a rigorously demonstrated theory but solely a hypothesis which appears to me more probable, more conformable to the laws of nature, and which appears to me to contain fewer forced explanations and fewer contradictions.
Circumstances have permitted me to give here but a general outline of the system and a glance at its consequences, but I propose to take up successively each point, to develop each in different memoirs, and I venture to assert in advance that the hypothesis which I propose explains in a very satisfactory and very simple manner the principal phenomena of physics and chemistry.
This attitude is consistent with the scientific method of Francis Bacon (1561-1626), which emphasized collection and classification of facts [Bacon 1620]. Bacon's approach was inductive, generalizing the laws of nature from a vast collection of observations. The thought of arriving at conclusions about nature by the process of elimination, however, is strange to the modern scientist.
I.e. the phlogiston theory, formulated by Georg Ernst Stahl (1660-1734; see portrait at the Edgar Fahs Smith collection, University of Pennsylvania) and relying as well on the work of Stahl's teacher, Johann Joachim Becher (1635-1682, see image at AEIOU, the Austrian Cultural Information System).
Those facts increased because Lavoisier continued to investigate combustion and respiration and to accumulate those facts. The Mémoires of the French Academy for 1777 include four other contributions by Lavoisier on details that the present communication generalizes: work on the combustion of phosphorus (pp. 65-78), the respiration of animals (185-94), the burning of candles in atmospheric air and in oxygen (195-204), and on the combination of matter of fire with volatile fluids (420-32).
Lavoisier's explanation of combustion did not adequately address the generation of heat and light. In fact, both light and heat (the latter under the name of caloric or matter of fire) appear in his list of elements (chapter 3). Today we understand heat and light to be forms of energy, not matter. At this stage, Lavoisier seems to regard "matter of fire" as encompassing heat, light, and flame.
For Priestley's work, which he communicated to Lavoisier, see the previous chapter. Lavoisier went on to carry out similar experiments. He reported on these experiments to the French Academy of Science in spring 1775 [Lavoisier 1775a], and revised his results before the final published version of that report appeared in 1778 [Lavoisier 1775b]. James Bryant Conant presents a detailed comparison of the original and revised version of Lavoisier's memoir and the influence on them of Priestley's work. [Conant 1957]
Robert Boyle had carried out an interesting set of experiments which demonstrated the difficulty of sustaining combustion even in the relatively poor vacua he was able to generate [Boyle 1672].
This key point illustrates how fruitfully suggestive a quantitative observation can be. Lavoisier first made this observation with regard to calcination [Lavoisier 1775a], but as he notes below, calcination is essentially a slow form of combustion. In his experiments on the calx of mercury (mercuric oxide, HgO), Lavoisier weighed the calx before heating it and carefully recovered the mercury produced by heating the calx. The weight of the original calx equals the weight of the mercury plus the weight of the "pure air." This relationship among the weights strongly suggests a similar relationship among the materials, namely that the calx is made up of mercury plus "pure air." Immediately, this picture is different from that of the phlogiston theory: the calx is a combination of the metal and something else (oxygen), rather than the metal being a combination of the calx and something else (phlogiston).
Here we see where Lavoisier got the idea that oxygen was an essential part of acids and why he named "pure air" oxygen, for the substance that increases the weight of the original combustible substance is oxygen. Lavoisier over-generalized, though: a great many products of oxidation are acidic, but it does not follow (and in fact it is not true) that all acids are the result of oxidation. Today we recognize the common strong acid hydrochloric acid (HCl) as a counterexample to Lavoisier's opinion, for it is an acid that contains no oxygen. Yet Lavoisier's opinion was so influential, that more than two decades passed between the isolation of chlorine and the recognition that chlorine was not a compound of oxygen and was in fact an element [Davy 1810].
I will observe here in passing that the number of acids is infinitely greater than we think. [--Lavoisier's note]
Lavoisier points out that there was no direct evidence for phlogiston. It was not unreasonable for the formulators of the phlogiston theory to hypothesize that all combustible bodies shared a common constituent which made them all combustible. Subsequent investigation, however, did not turn up any such substance. Indeed, the essential property attributed to the hypothetical substance phlogiston was its combustibility. The phlogiston hypothesis could stand as a working hypothesis as long as it was not contradicted by a more compelling hypothesis, one for which there was more evidence or better evidence; Lavoisier goes on to propose just such a hypothesis.
By igneous fluid, Lavoisier means the hypothetical "matter of fire." Igneous means of or related to fire. Fluid means a material which can flow, so a fluid encompasses both liquids and gases. In rejecting the phlogiston theory Lavoisier has obviously not rejected the concept of "matter of fire."
By aeriform liquid, Lavoisier means a fluid like air, i.e., a gas. He believes gases to be compounds of heat with ordinary solids or liquids. Since this explanation is based on the mistaken notion that heat ("igneous fluid") is a material, it is an incorrect explanation. It was not, however, an unreasonable hypothesis: for example, it provides an explanation for the observed phenomena of evaporation and condensation as well as the fact that gases are elastic (i.e., compressible, but only with application of force). Dalton later incorporated this idea into his atomic hypothesis, picturing atoms in a gas to be ordinary atoms coated by a shell of heat which kept the atoms from staying close together [Dalton 1808].
Base here has more of a non-technical meaning than the current chemical meaning of a substance which reacts with acids. Lavoisier envisions oxygen (Priestley's dephlogisticated air) to be a combination of heat and another substance (called the base of the air in the next paragraph). When this combination, oxygen, meets with something that unites more strongly with the base than does heat (i.e., has a greater affinity for the base than does heat); that something will combine with the base and release the heat. In this picture, combustion is the combination of a combustible substance with the base of the air, accompanied by the release of heat. This is correct in that combustion is a combination of a combustible substance with oxygen accompanied by the release of heat (except that heat is a form of energy and not a material "matter of fire").
Fixed air, acid of chalk, and mephitic acid are all the same thing, now called carbon dioxide (CO2). The various names arise from its various properties. For example, the gas could be "fixed" (i.e., immobilized) by its incorporation into a solid if it was bubbled through an alkaline solution. It was called acid of chalk because it is itself a weakly acidic substance which can be produced by the reaction of a stronger acid with chalk (calcium carbonate, CaCO3). This gas was called mephitic (noxious or poisonous) because it does not support respiration.
Carbon dioxide is a product of the burning of carbon-containing compounds. Lavoisier says that this combustion is still the combination of a combustible material with the base of the air, accompanied by the release of heat. Only here, some of the heat stays with the product, making it a gas. The burning of charcoal, then, produces as much gas ("fixed air") as the oxygen it uses up.
This paper represents an interesting stage in the evolution of the concept "matter of fire" from phlogiston to caloric. Phlogiston was thought to be a principle of fire or combustibility which resided in substances which could be burned. Stahl had called it "the corporeal fire, the essential fire material," which was also implicated in phenomena of a "finely divided and invisible fire, namely, heat" [Stahl 1718]. In this paper, Lavoisier still considers the "matter of fire" a hypothetical material associated with combustion, but now it is supposed to reside in oxygen. By the time of his Elements of Chemistry, he distinguishes the phenomenon of light, at least, from that of heat, although he associates each with material fluids. Lavoisier coins the term caloric for what he called igneous fluid or matter of heat. [Lavoisier 1789, p. 5] On this matter, J. H. White has observed, "In many respects caloric' was really 'phlogiston' reappearing, much curtailed and humbled in scope it is true, under another name." [White 1932, p. 155] Early 19th century work which treats heat as a fluid and distinguishes it still further from fire (e.g., Dalton 1808, chapter I). And later in the 19th century, heat is recognized as a form of energy, not a material at all.
If one substitutes "heat" for "fire," this paragraph could easily be confused with one written by Dalton more than 30 years later [Dalton 1808]. Lavoisier's observation in the previous paragraph that oxygen is the only material which supports combustion seems to contradict, rather than support, the assertion of this paragraph. Granted, this paragraph is consistent with Lavoisier's assertion that combustible solids and liquids contain little or no "matter of fire"; however, the fact that most gases do not support combustion argues against the assertion that all gases contain "matter of fire" in great abundance.
Lavoisier seems to escape the contradiction mentioned in the previous footnote by implying that gases other than oxygen hold onto their "matter of fire" more tightly than does oxygen. If that were true, then one would expect those other gases to condense with much more difficulty than oxygen, for condensation in Lavoisier's mind also involves a vapor giving up its "matter of fire" (i.e., heat).
Lavoisier argues here, plausibly though incorrectly, that it is more reasonable to suppose a light, active, and lively substance like "matter of fire" to be found in gases (also light) than in substances like diamond or metals.
Lavoisier is widely but inaccurately credited with disproving the phlogiston theory because his experiments showed metals to weigh less than their supposedly simpler calces. But he was not the first to notice that metals gain weight on calcination. See, for example, Rey 1630.
Lavoisier takes up this subject in greater detail in Lavoisier 1777a. Respriation is a slower and milder version of combustion than even calcination, so oxygen is required to sustain respiration as well. In respiration, oxygen from the air inhaled by an animal oxidizes molecules from the animal's food. Carbon dioxide is given off as a product of respiration, just as it is a product of combustion of carbon-containing substances. And respiration is, as Lavoisier suspects, the source of animal heat.