Antoine Laurent Lavoisier (1743-1794)

Memoir on the Nature of the Principle which Combines with Metals during their Calcination and which Increases their Weight

Mémoires de l'Académie Royale des Sciences for 1775, 520-6 (published in 1778).

Are there different species of air? Is it sufficient that a body be in a durable state of expansibility in order to be a species of air? Finally, are the different airs which occur in nature or which we may produce separate substances or merely modifications of the air of the atmosphere? Such are the principal questions which encompass the plan which I have formed and whose successive development I propose to bring before the eyes of the Academy. But the time devoted to our public meetings does not permit me to treat any of these questions extensively, and I will confine myself today to a particular case and limit myself to showing that the principle which combines with metals during their calcination, which increases their weight and constitutes them in the state of a calx, is nothing other than the most salubrious and purest portion of the air and such that, if the air, after having engaged in a metallic combination, becomes free again, it appears in an eminently respirable state more capable than the air of the atmosphere of sustaining ignition and combustion.

The majority of metallic calces are not to be reduced, that is, returned to the metallic state, without the intermediate contact of a carbonaceous material or any substance whatsoever containing what we call phlogiston. The charcoal which is used is completely destroyed in this operation if it be present in suitable proportion; whence it follows that the air which is evolved in metallic reductions with carbon is not a simple substance but in some manner is the result of the combination of the elastic fluid disengaged from the metal and that disengaged from the carbon. Therefore the fact that this fluid is obtained as fixed air gives us no right to conclude that it existed in this form in the metallic calx before its combination with the carbon.

These considerations showed me that in order to clear up the mystery of the reduction of metallic calces it would be necessary to experiment with those calces which are reducible without the addition of anything. The calx of iron offered me this property and actually, of all those calces, either natural or artificial, which we have exposed at the foci of the large burning glasses either of the Regent or of Mr. Trudaine, there have been none which have not been completely reduced without addition.

I tried, consequently, to reduce by means of a burning glass several species of the calx of iron under large glass bells inverted in mercury, and I succeeded in disengaging by this means a large quantity of elastic fluid. But at the same time this elastic fluid became mixed with the common air contained in the bell, and this circumstance threw much uncertainty on my results, so that none of the tests which I conducted upon this air were perfectly conclusive and it was impossible for me to be certain whether the phenomena I obtained arose from the common air, from that disengaged from the calx of iron, or from the combination of the two. The experiments having failed of fully filling my purpose, I omit their details here; they will, however, find their natural place in other memoirs.

As much as these difficulties arose from the nature of iron itself, from the refracory nature of its calces, and from the difficulty of reducing them without addition, I regarded them as insurmountable and therefore thought that I ought to direct my attention to another species of calx, more easily treatable and being, like the calces of iron, reducible without addition. Precipitated mercury per se, which is nothing else than a calx of mercury, as several authors have already advanced and as will appear even more convincingly by the reading of this memoir, precipitated mercury per se, as I said, appeared to me to be completely appropriate for the object which I had in view, for everyone knows today that this substance is reducible without addition at a very medium degree of heat. Although I have repeated a great many times the experiments which I am about to describe, I have not thought it appropriate to give the details of each of them here for feat of extending the memoir too far, and consequently I have combined into a single account the circumstances pertaining to many repetitions of the same experiment.

First, to assure myself that precipitated mercury per se was a genuine metallic calx, that it gave the same results, the same species of air on reduction according to the ordinary method (that is, to use the customary expression, with the addition of phlogiston), I mixed an once of this calx with 48 grains of powdered charcoal and introduced the mixture into a little glass retort of 2 cubic pouces or more capacity. This I placed in a reverberatory furnace of proportionate size. The neck of this retort was about a pied and 3 to 4 lignes in diameter and was bent in various places by means of an enameler's lamp in such a manner that its end was disposed beneath an ample glass bell filled with water and inverted in a tub of the same. The apparatus which is here before the eyes of the Academy will suffice to illustrate its operation. This apparatus, simple as it is, is even more accurate in that it has neither joints nor lute nor any passage through which the air may escape.

As soon as a fire was placed beneath the retort and the first effects of the heat felt, the common air which it contained expanded and some little of it passed into the bell. However, in view of the small volume of the empty part of the retort, this air made no sensible error, and its quantity taken at the most can scarcely amount to a cubic pouce. As the retort is heated further the air is evolved with much speed and rises through the water in the bell. The operation did not last for more than three-quarters of an hour, the fire being kept up during this interval. When all the calx of mercury had been reduced and the air ceased to come forth, I marked the height of the water in the bell and found that the quantity of air evolved had been 64 cubic pouces without counting that which was unavoidably absorbed in traversing the water.

I submitted this air to a large number of tests, the details of which I omit, and found that 1) it can, by shaking, combine with water and give to the water all the properties of acidulated, gaseous, or aerated waters such as those of Seltz, Pouges, Bussang, Pirmont, etc.; 2) it kills in some seconds animals which were placed in it; 3) candles and all combustible bodies in general are extinguished in an instant; 4) it precipitates lime water; 5) it combines with great ease with either fixed or volatile alkalies, depriving them of their causticity and making them capable of crystallizing. All of these properties are precisely those of that species of air known under the name of fixed air which I obtained by the reduction of minium by powdered charcoal, which calcareous earths and effervescent alkalis evolve in combining with acids, and which vegetable materials evolve in fermenting. It was thus established that precipitated mercury per se gives the same products as other metallic calces when reduced with the addition of phlogiston, and that it belongs, therefore, in the general class of metallic calces.

It then only remained to examine this calx alone, to reduce it without adding anything, to see if some elastic fluid were evolved from it, and, supposing there were, to determine its nature. To this end I placed in a retort of 2 cubic pouces capacity 1 once of precipitated mercury per se alone, arranged the apparatus in the same manner as in the preceding experiment, and operated so that all the circumstances would be exactly the same. The reduction took place this time with a little more difficulty than when charcoal was added; more heat was required, and there was no sensible change until the retort began to become slightly red. Then the air was evolved little by little, passed into the bell, and, holding the same degree of fire during two and one-half hours, all the mercury was reduced.

The operation completed, there was found, on the one hand, partly in the neck of the retort, and partly in a glass vessel which I placed beneath the water under the exit of the retort, 7 gros and 18 grains of fluid mercury, and on the other hand, the quantity of the air which had passed into the bell was found to be 78 cubic pouces; whence it follows that by supposing that the whole loss of weight should be attributed to the air, each cubic pouce should weigh a little less than two-thirds of a grain--a value not far removed from that of common air.

After having thus fixed the first results, I had only to submit the 78 cubic pouces of air which I had obtained to all the tests necessary to determine its nature, and I found with much surprise

  1. That it would not combine with water on shaking
  2. That it did not precipitate limewater but only gave it a nearly imperceptible turbidity
  3. That it failed to unite at all with fixed or volatile alkalis
  4. That it failed entirely to diminish the causticity of these
  5. That it could be used again to calcine metals
  6. {cut1}Finally, that it had none of the properties of fixed air.

In contrast to the latter, animals did not perish in it and it seemed more suitable to their respiration. Candles and inflamed materials were not only not extinguished, but the flame widened in a very remarkable manner and shed much more light and brilliancy than in common air. Charcoal burned therein with a brilliance nearly like that of phosphorus, and all combustible materials in general were consumed with astonishing rapidity. All these circumstances have fully convinced me that this air, far from being fixed air, is in a more respirable, more combustible state and in consequence is more pure even than the air which sustains us.

It appears to be proved from the above that the principle which combines with and increases the weight of metals when they are calcined is nothing other than the purest portion of the air itself which surrounds us and which we breathe--this it is which in calcination passes from the expansible state to the solid one. If, then, this principle is obtained in the form of fixed air in all metallic reductions in which carbon is used, it follows that this is due to the combination of this latter with the pure portion of the air, and it is very probable that all metallic calces would, like mercury, give only eminently respirable air if we could reduce them all as we do precipitated mercury per se.

All that has been said of the air of metallic calces applies naturally to that which is obtained from niter by explosion. It is known from a number of experiments already published, and which I have in greatest part repeated, that the major part of this air is in the state of fixed air, is deadly to animals which breathe it, and has the property of {cut2} uniting easily with lime and the alkalis, rendering them mild and capable of crystallizing. But since, at the same time, the explosion of niter takes place only with the addition of carbon or any substance which contains phlogiston, one can hardly doubt that, under these circumstances, eminently respirable air is converted into fixed air. From this it would follow that the air combined in niter which produces the terrible explosions of gunpowder is the respirable portion of the air of the atmosphere deprived of its expansibility and is one of the constituent principles of nitric acid.

Since charcoal disappears completely in the revivification of the calx of mercury, and since one retrieves in this operation only mercury and fixed air, one is forced to conclude that the principle to which has been given till now the name of fixed air is the result of the combination of the eminently respirable portion of the air with charcoal. I propose to develop this in a more satisfying manner in a series of memoirs which I shall give on the topic.

[nothing other than ... ignition and combustion.] neither one of the constituent parts of the air, nor a particular acid distributed into the atmosphere, that it is the air itself entire without alteration, without decomposition even to the point that if one sets it free after it has been so combined it comes out more pure, more respirable, if this expression may be permitted, than the air of the atmosphere and is more suitable to support ignition and combustion.

[which I obtained ... metallic calces.] or mephitic air such as is obtained from all metallic calces by the addition of charcoal such as is set free from fermenting matters. It was thus unquestionable that mercurius calcinatus per se should be included in the category of metallic calces.

[its nature] what state it was in

{cut1}that it was diminished like common air by an addition of a third of nitrous air;

[, far from being ... more combustible state] was not only common air but that it was more respirable, more combustible

[combination of this latter with the pure portion of the air] charcoal itself

[eminently respirable air] common air

{cut2} precipitating lime water and of

[one can hardly doubt ... give on the topic.] it is very probable that in this circumstance also common air is converted into fixed air; from which it follows that the air combined with niter which produces the terrible explosions of gunpowder is common atmospheric air deprived of its elasticity.
From the fact that common air changes to fixed air when combined with charcoal it would seem natural to conclude that fixed air is nothing but a combination of common air and phlogiston. This is Mr. Priestley's opinion and it must be admitted that it is not without probability; however, when one looks into the facts in detail, contradictions arise so frequently I feel it necessary to ask natural philosophers and chemists still to suspend judgment; I hope to be soon in a position to communicate the reasons for my doubts.