Observations sur la Physique23, 452-5 (1783); translation by Carmen Giunta
From the year 1777, M. Lavoisier and M. Bucquet, in a series of experiments carried out jointly, noticed that burning large amounts of inflammable air, obtained from the dissolution of iron in vitriolic acid, formed not the slightest amount either of fixed air nor of any other acid whatsoever.
M. Cavendish made the same observation in England. Furthermore, he observed that if one operates in dry vessels a discernible quantity of moisture is deposited on the inner walls.
Since the verification of this fact was of great significance to chemical theory, M. Lavoisier and M. de la Place proposed to confirm it in a large-scale experiment; and in order to give it greater authority, they engaged several Members of the Academy to assist in it. They prepared a sort of double-tubed lamp for inflammable air, one tube carrying inflammable air and the other dephlogisticated air. The two orifices through which the airs passed were severely restricted, to make the combustion very slow, and they were proportioned in such a way as to supply the amounts of the respective airs needed for combustion. The glass bell into which the double tube led was immersed in mercury, and had no communication with the exterior air. Last July or August M. Lavoisier gave the Academy a detailed description of this apparatus. The quantity of inflammable air burned in this experiment was about thirty pints [pintes] and that of dephlogisticated air from fifteen to eighteen.
As soon as the two airs had been lit, the wall of the vessel in which the combustion took place visibly darkened and became covered by a large number of droplets of water. Little by little the drops grew in volume. Many coalesced together and collected in the bottom of the apparatus, where they formed a layer on the surface of the mercury.
After the experiment, nearly all the water was collected by means of a funnel, and its weight was found to be about 5 gros, which corresponded fairly closely to the weight of the two airs combined. This water was as pure as distilled water.
A short time later, M. Monge addressed to the Academy the result of a similar combustion, carried out at Mézières, with a totally different apparatus and which was perhaps more accurate. He determined with great care the weight [pesanteur] of the two airs, and he likewise found that in burning large quantities of inflammable air and dephlogisticated air one obtains very pure water and that its weight very nearly approximates the weight of the two airs used. Finally, it was reported in a letter written from London by M. Blagden to M. Bertholet, that M. Cavendish recently repeated the same experiment by different means and that when the quantity of the two airs had been well proportioned, he consistently obtained the same result.
It is difficult to refuse to recognize that in this experiment, water is made artificially and from scratch, and consequently that the constituent parts of this fluid are inflammable air and dephlogisticated air, less the portion of fire which is released during the combustion.
Meanwhile, before admitting a consequence so remote from all received ideas, M. Lavoisier thought it necessary to multiply the proofs and above all, after having established by means of composition the nature of the constituent parts of water, to set himself to the task of regenerating them by means of decomposition.
With this purpose, he filled a crystal bowl with mercury, inverted it in a vessel filled with mercury, and introduced a small portion of water and of iron filings, very pure and not rusted. From the first day, the iron began to lose a part of its metallic luster; it was calcined and converted in part to rust. At the same time it released a quantity of inflammable air in proportion to the quantity of dephlogisticated air which had been absorbed by the iron, as judged by the increase in weight which the filings had aquired after being dried. Thus water, in this experiment, is decomposed into two distinct substances, dephlogisticated air which unites with the iron and converts it to a calx, and inflammable air which remains separate. On the other hand, when one reunites and recombines these same two substances, one recomposes water. Thus one is led still more nearly inevitably to conclude that water is not a simple substance at all, not properly called an element, as had always been thought.
It is easy to imagine that this discovery must have opened to M. Lavoisier a vast field of experiments, and they led him to believe that a great number of phenomena which were attributed to the decomposition of bodies were due to that of water. The dissolution of metals in acids supply striking examples. In almost all these operations, the metal begins to be calcined before dissolving; that is to say, that it combines with a certain quantity of dephlogisticated air, a different amount according to the nature of the metal. He maintains he obtained proof through these experiments, many of which he performed jointly with M. de la Place, that in all the dissolutions of metal in vitriolic acid, the dephlogisticated air needed for the calcination of the metal is not supplied by the acid but by the water, and that at the same time inflammable air, which is one of its constituents, becomes freed and is released in its aeriform state.
In contrast, in the dissolutions of metals in nitrous acid, the greater part of the dephlogisticated air is supplied by the acid, and the water only contributes a small portion. He reports that he has not yet attempted any research on dissolutions by marine acid, because of some difficulties which attend this kind of combination, of which he promises to give an account.
After having followed the effects of the decomposition of water in the dissolution and calcination of metals, M. Lavoisier gave an account of several experiments which he undertook with the same aim on the fermentation of wine. Although it happens that he has not yet obtained an absolutely decisive result, he thinks it correct meanwhile to suspect and even to believe that the formation of the vinous ingredient is due to the decomposition of water. In this operation, the dephlogisticated air from water unites with the carbonaceous part of the sugary substance and forms fixed air, which is released thoughout the duration of the fermentation. At the same time, inflammable air, modified and combined with another portion of water by means of an intermediate as yet unknown forms the spiritous part. Likewise, in following the operation of plants, he appears to be brought to believe that the formation of combustible plant material is due to the inflammable air contained in water. Doubtless these assertions appear perhaps [sic: sans doute ... peut-être] hazarded at first glance; however, M. Lavoisier promised further detail beyond the evidence contained in this first Memoir. He finished his Memoir with this modest conclusion: that if the decomposition of water in a multitude of operations of Nature and Art is not rigorously demonstrated, it is at least infinitely probable.