A. Lavoisier

(Read to the Academie des Sciences, Easter, 1775.)

Are there different kinds of air?.... Are the different airs that nature offers us, or that we succeed in making, exceptional substances, or are they modifications of atmospheric air? Such are the principal subjects embraced in my scheme of work, the development of which I propose to submit to the Academy. But since the time devoted to our public meetings does not allow me to treat any one of these questions in full, I will confine myself to-day to one particular case, and will only show that the principle which unites with metals during calcination is nothing else than the healthiest and purest part of air, so that if air , after entering into combination with a metal, is set free again, it emerges in an eminently respirable condition, more suited than atmospheric air to support ignition and combustion.

The majority of metallic calces are only reduced, that is to say, only return to the metallic condition, by immediate contact with a carbonaceous material, or with some substance containing what is called phlogiston. The charcoal that one uses is entirely destroyed during the operation when the amount is in suitable proportion, whence it follows that the air set free from metallic reductions with charcoal is not simple; it is in some way the result of the combination of the elastic fluid set free from the metal and that set free from the charcoal; thus, though this fluid is obtained in the state of fixed air it is not justifiable to conclude that it existed in this state in the metallic calx before its combination with the carbon.

These reflections made me feel how essential it was-- in order to unravel the mystery of the reduction of metallic calces -- to perform all my experiments on calces which can be reduced without addition of charcoal....

Precipitated mercury, which is nothing else than a calx of mercury, as several authors have suggested and as this memoir will further show, precipitated mercury, I repeat, seemed to me suitable for the object I had in view: for nobody to-day is unaware that this substance can be reduced without addition of charcoal at a very moderate degree of heat. Although I have repeated many times the experiments I am going to quote, I have not thought it suitable to give particular details of any of them here, for fear of occupying too much space, and I have therefore combined into one account the observations made during several repetitions of the same experiment.

In order to be sure that precipitated mercury was a true metallic calx, that it gave the usual results and the usual kind of air on reduction by the ordinary method, that is to say, using the recognized expression, by the addition of phlogiston, I mixed one ounce of this calx with 48 grains of powdered charcoal, and introduced the whole into a little glass retort of at most two cubic inches capacity, which I placed in a reverberatory furnace of proportionate size. The neck of this retort was about a foot long, and three to four lines in diameter; it had been bent in a flame in different places and its tip was such that it could be fixed under a bell-jar of sufficient size, filled with water, and turned upside down in a trough of water...

As soon as a flame was applied to the retort and the heat had begun to take effect, the ordinary air contained in the retort expanded, and a small quantity passed into the bell-jar; but in view of the small size of the part of the retort that remained empty, this air could not introduce a sensible error, and at the most it could scarcely amount to a cubic inch. When the retort began to get hotter, air was very rapidly evolved and bubbled up through the water into the bell-jar; the operation did not last more than three-quarters of an hour, the flame being used sparingly during this interval. When the calx of mercury was reduced and air was no longer evolved, I marked the height at which the water stood in the bell-jar and found that the air set free amounted to 64 cubic inches, without allowing for the volume necessarily dissolved in the water.

I submitted this air to a large number of tests, which I will not describe in detail, and found (1) that it combined with water on shaking and gave to it all the properties of acidulated, gaseous, or aerated water such as those of Seltz, Bougues, Bussang, Pyrmont, etc.; (2) that animals placed in it died in a few seconds; (3) that candles and all burning bodies were instantly extinguished therein; (4) that it precipitated lime water; (5) that it combined very easily with fixed or volatile alkalis, removing their causticity and giving them the power of crystallizing. All these are precisely the qualities of the kind of air known as fixed air, such as I obtained by the reduction of minimum by powdered charcoal, such as is set free from calcareous earths and effervescent alkalis by their combination with acids, or from fermenting vegetable matters, etc. It was thus certain that precipitated mercury gave the same products as other metallic calces on reduction in the presence of phlogiston and that it could consequently be included in the general category of metallic calces.

It remained to examine this calx alone, to reduce it without addition, to see if some elastic fluid was still set free, and if so, to determine the nature of such fluid. With this in view, I put into a retort of the same size as before ( two cubic inches) one ounce of precipitated mercury alone; I arranged the apparatus in the same way as for the preceding experiment, so that all the circumstances were exactly the same; the reduction was a little harder to bring about than when charcoal was present; it required more heat and there was no perceptible effect till the retort began to get slightly red-hot; then air was set free little by little, and passed into the bell-jar, and by keeping up the same degree of heat for 2 1/2 hours, all the mercury was reduced.

The operation completed, I found 7 gros, 18 grains of liquid mercury, some in the neck of the retort and some in a glass vessel I had placed at the tip of the retort under the water; the amount of air in the bell-jar was found to be 78 cubic inches; from this follows, if the loss of weight of the mercury is attributed to the loss of this air, that each cubic inch must weigh a little less than two-thirds of a grain, which does not differ much from the weight of ordinary air.

Having established these results, I hastened to submit the 78 cubic inches of air I had obtained to all the tests which could indicate its nature, and found, much to my surprise:

(1) that it did not combine with water on shaking;
(2) that it did not precipitate lime water, but only caused in it an almost imperceptible turbidity;
(3) that it entered into no compounds with fixed or volatile alkalis;
(4) that it did not in the least diminish their causticity;
(5) that it could be used again for the calcination of metals:
(6) in short, that it had none of the properties of fixed air:

far from causing animals to perish like fixed air, it seemed on the contrary more suited to support their respiration; not only were candles and burning objects not extinguished, but the flame increased in a very remarkable manner: it gave much more light than in common air; charcoal burned with a flash almost like that of phosphorus, and all combustible bodies were consumed with astonishing speed. All these circumstances fully convinced me that this air, far from being fixed air, was in a more respirable and combustible, and therefore in a purer condition, than even the air in which we live.

This seems to prove that the principle which unites with metals when they are calcined and causes them to increase in weight is nothing else than the purest part of the air which surrounds us, which we breathe, and which during calcination passes from a condition of expansibility to that of solidity; if it is obtained in the form of fixed air from metallic reductions in which charcoal is employed, this is due to the combination of the charcoal with the pure part of the air, and it is very probable that all metallic calces would give, like that of mercury, only this eminently respirable air, if one could reduce them all without addition, as precipitated mercury is reduced...


A. Lavoisier

(Read to the Academie des Sciences, 3rd May, 1777.)

Of all the phenomena of animal economy, none are more striking, nor more worthy of attention from physicists and physiologists than those accompanying respiration. If, on the other hand, we know little of the object of this singular function, we know, on the other hand, that it is so essential to life that it cannot be suspended for any time without exposing the animal to danger of immediate death.

Air, as everyone knows, is the agent or more exactly the subject of respiration, but, at the same time, all kinds of air, or more generally all kinds of elastic fluids, are not adapted to maintain it, and there are a large number of airs that animals cannot breathe without perishing at least as promptly as if they did not breathe at all.

The experiments of certain physicists and above all those of MM. Hale and Cigna first began to cast some light on this important matter; since then, M. Priestly, in a work published in London last year, has pushed back the limits of our knowledge yet further, and he has sought to prove, by very ingenious , very delicate, and very novel experiments, that the respiration of animals has the property of phlogisticating the air, like the calcination of metals and several other chemical processes, and that air only ceases to be respirable at the moment when it is overcharged and in some way saturated with phlogiston.

However probable the theory of the celebrated physicist may have appeared at first sight, however numerous and well-performed the experiments on which he sought to establish his theory, I must own that I have found it to be contradicted by so many phenomena that I have thought myself justified in calling it in question; I have consequently worked on another plan and I have been irresistibly led by my experiments to conclusions quite other than his. I will not stop to discuss in detail each of M. Priestley's experiments, nor to demonstrate how they all prove to be in favour of the opinion I am going to expound in this memoir; I will content myself with quoting those which are necessary for my purpose...

I enclosed 50 cubic inches of common air in a suitable apparatus, which it would be difficult to describe without illustrations; I introduced into this apparatus 4 ounces of very pure mercury and proceeded to calcine it, keeping it for twelve days at the heat almost sufficient to make it boil.

Nothing noteworthy happened during the first day; the mercury, though not boiling, was in a state of continual evaporation... On the second day, red specks began to appear on the surface of the mercury, and they increased in size and volume daily; finally, at the end of twelve days, having let the fire out and allowed the vessel to cool, I observed that the air it had contained was diminished by 8 to 9 cubic inches, that is to say, by about a sixth of its volume; at the same time there had been formed a considerable quantity, approximately 45 grams, of precipitated mercury, or calx of mercury.

The air thus diminished in volume did not precipitate lime water, but it extinguished flames, caused animals placed in it to die in a short while, gave practically no red vapours with nitrous air, nor was perceptibly diminished by it -- in a word, it was in an absolutely mephitic condition.

The experiments of M. Priestley and myself have made clear that precipitated mercury is nothing else than a compound of mercury with about one twelfth of its own weight of a kind of air that is better and more respirable, if I may use such a word, than common air; it appears then to be proved that in the foregoing experiment the mercury absorbed the better and more respirable part of the air during calcination, leaving behind only the mephitic or non-respirable part; the following experiment confirmed this truth yet further.

I carefully collected the 45 grains of calx of mercury formed in the preceding calcination; I put them in a very small glass retort whose neck, doubly bent, was fixed under a bell-jar full of water, and I proceeded to reduce it without adding anything.

By this operation I recovered almost the same amount of air that had been absorbed by the calcination, that is to say, 8 to 9 cubic inches, and on combining these 8 to 0 cubic inches with the air vitiated by the calcination of mercury, I restored this air exactly enough to its state before calcination, i.e., to the state of common air: the air thus restored no longer extinguished flames, no longer caused the death of animals breathing it, and finally was almost as much diminished by nitrous air as the air of the atmosphere.

Here is the most complete kind of proof to which one can attain in chemistry, the decomposition and recomposition of air, and from this one may evidently conclude (1) that five-sixths of the air we breathe are incapable of supporting the respiration of animals, or ignition and combustion; (2) that the surplus, i.e., one fifth only of the volume of atmospheric air is respirable; (3) that when mercury is calcined, it absorbs the healthy part of the air, leaving only the noxious part; (4) that on bringing together the two parts of the air thus separated, the respirable part and the noxious part, one reproduces air similar to that of the atmosphere.

The preliminary facts concerning the calcination of metals lead us to simple conclusions concerning the respiration of animals, and as air which has for some time served to support this vital function has much in common with that in which metals have been calcined, knowledge of the one may naturally be applied to the other...

Air which had been breathed by an animal till it died became very different from atmospheric air; it precipitated lime water; it extinguished flames; it was no longer diminished by nitrous air; another animal introduced into it only lived some seconds; it was entirely mephitic, and in this respect appeared similar enough to that which remained after the calcination of mercury.

However a more thorough examination showed me two very remarkable differences between these two airs, I mean between that which had been used for the calcination of mercury and that which had been used for support respiration: firstly, the diminution of volume was much less in the latter than in the former; secondly, air from respiration precipitated lime water, while air from calcination caused no change therein.

This difference, on the one hand, between these two airs, and , on the other hand, the close analogy which they showed in many respects, led me to assume that two processes were confused in respiration of which I probably yet knew but one, and to remove my uncertainties on this matter, I made the following experiment.

I caused 12 cubic inches of air vitiated by respiration to pass into a bell-jar full of mercury, turned upside down in a trough full of mercury, and I introduced also a thin layer of fixed caustic alkali; I might have used lime water instead, but the volume which it would have been necessary to employ would have been too large and would have been prejudicial to the success of the experiment.

The effect of the caustic alkali was to occasion a diminution of nearly one-sixth in the volume of the air; at the same time the alkali partly lost its causticity; it acquired the property of effervescing with acids, and crystallized even under the bell-jar in very regular rhomboids; properties which one knows cannot be communicated to it except by its combination with the kind of air or gas known as fixed air, which I shall henceforward call "gaseous acid of chalk"; it follows that air vitiated by respiration contains almost one-sixth of a gaseous acid exactly similar to that obtained from chalk.

Far from being restored to the state of common air, the air thus feed from its fixable portion by caustic alkali had more resemblance to air used for the calcination of mercury, or rather was one and the same thing; like the latter, it caused animals to die, extinguished flames; in fact, none of the experiments I made to compare these two airs enabled me to perceive the slightest difference between them.

But air which has been used for the calcination of mercury is nothing else, as we have seen above, than the mephitic residue of atmospheric air, the respirable part having combined with the mercury during the calcination; so that air which has been used for respiration, when it has been deprived of the gaseous acid of chalk that it contains, is likewise nothing but the residue of common air deprived of its respirable portion; and, in fact, having added to this air about a quarter of its volume of condition and made it as suitable as ordinary air for respiration, or for the support of combustion in the same way as I had restored air which had been vitiated by the calcination of metals.

It follows from these experiments that to change air vitiated by respiration back to the state of ordinary respirable air, it is necessary (1) to remove from this air by means of lime or a caustic alkali the portion of gaseous acid of chalk that it contains; (2) to restore to it a quantity of respirable or dephlogisticated air equal to that which it has lost. Respiration, by a necessary consequence, must effect the reverse of these two changes, and I find myself led to two equally probable explanations of this, between which experiment does not yet allow us to decide.

For from what goes above one may conclude that one of two things occurs as a result of respiration: either the respirable air contained in the air of the atmosphere is converted into gaseous acid of chalk during its passage through the lung; or there is an exchange in this organ, respirable air being absorbed and an almost equal volume of gaseous acid of chalk being given up by the lung.....


A. Lavoisier

(Communicated to the Academie des Sciences, 1777.)

I have established in the foregoing memoirs that the air of the atmosphere is not a simple substance, an element, as the air we breathe is composed of respirable air to the extent of only one quarter and that the remainder is a noxious gas (probably itself of a complex nature) which cannot alone support the life of animals, or combustion or ignition. I feel obliged, consequently, to distinguish four kinds of air or air-like fluids.

Firstly, atmospheric air, that in which we live, which we breathe, etc.

Secondly, pure air, respirable air; that which forms only a quarter of atmospheric air and which M. Priestley has very wrongly called dephlogisticated air.

Thirdly, the noxious air which makes up three quarters of atmospheric air and whose nature is still entirely unknown to us.

Fourthly, fixed air, which I shall call henceforward, following M. Bucquet, by the name of acid of chalk.