Such then is a way to decompose the acid of nitre and demonstrate the existence in it of a pure air and (if I may he allowed to use this expression) more an air than ordinary air. But the complement of the proof was, after having decomposed the acid, to succeed in re-compounding it out of the same materials, and that is what 1 have done.
[Lavoisier here inserts some preliminary remarks about the nature of nitrous air, and then describes his experiment as follows:]
I filled with water a tube which was closed at one end and which was marked off along its length by equal divisions of volume. I inserted this tube, thus filled with water, in another vessel, likewise filled with water; I let into it seven and one-third parts of nitrous air and mixed with this at the same time four parts of air purer than ordinary air, which I had measured out in another separate tube. At the moment of mixture, the eleven and a third parts of air occupied 12 to 13 measures, but, a moment later, the two airs mingled and combined, very red vapors of spirits of fuming nitre were formed, which were at once condensed by the water, and in a few seconds the eleven and a third parts of air were reduced to about a third of a measure; that is to say, to about the thirty-fourth part of their original volume.
The water contained in the tube was sensibly acid at the end of this experiment, or, rather, it was a weak acid of nitre; when I treated it with alkali I got from it by evaporation real nitre. . . . After having shown that one can separate and combine again the principles of the acid of nitre, it remains for me to show that the same can be done with materials not all taken from the acid of nitre. Instead of the purest air, or that drawn from the red precipitate of mercury, one may use the air of the atmosphere; but much more of it will have to be used, and instead of the four parts of pure air which are sufficient to saturate seven and one third parts of nitrous air, one will have to use nearly sixteen of common air; all the nitrous air is, in this experiment, as in the preceding one, destroyed or rather condensed; but this is not the case with common air; not more than a fifth or a fourth of it is absorbed, and what remains is no longer able to support the flame of a candle or to support respiration in animals. It seems proved by this that the air which we breathe contains only a fourth part of real air; that this real air is in our atmosphere mixed with three or four parts of a harmful air, a sort of choke-damp, which would cause the death of the majority of animals if it were present in a little greater quantity. The injurious effects on the air of vapor of charcoal and of a large number of other emanations prove how near this fluid is to the point beyond which it would be fatal to animals. I hope to soon be in a position to discuss this idea and to place before the Academy the experiments on which it is based.
- I pass over the tentative efforts by which I came to know the exact proportion.