In the case of other substances mere exposure to the air, aided by a moderate degree of heat, suffices to bring about the combination, and this is what happens to all vegetable substances capable of passing on to acid fermentation. In the greater number of cases one has to resort to the science of affinities and to employ the acidifying principle already united in another compound.
The example which I am going to give to-day is of this last sort, and I shall take it from an experiment, well known for several years, following the memoirs of M. Bergman. It is the formation of the acid of sugar. This acid, in accordance with the experiments which I am going to recount, seems to me to be nothing else than sugar combined with the acidifying principle or oxygen, and I propose to show in order in different memoirs that we can combine this same principle with the substance composing animals' horns, with silk, with animal lymph, with wax, with essential oils, with extracted oils, manna, starch, arsenic, iron and probably with a great many other substances of the three kingdoms. "We can thus turn all these substances into genuine acids.
Before entering on the material to be presented, I beg the Academy to recall that the acid of nitre, as shown by the experiments which I have before described, and which I have repeated in your presence, is the result of the union of nitrous air with the purest air or acidifying principle; that the proportion of these two principles varies in the different kinds of acid of nitre, the one which gives off fumes, for instance, containing a superabundance of nitrous air.
