We have, in Chap. V. and VIII. examined the products resulting from the combustion of the four simple combustible substances, sulphur, phosphorus, charcoal, and hydrogen: We have shown, in Chap. X that the simple combustible substances are capable of combining with each other into compound combustible substances, and have observed that oils in general, and particularly the fixed vegetable oils, belong to this class, being composed of hydrogen and charcoal. It remains, in this chapter, to treat of the oxygenation of these compound combustible substances, and to show that there exist acids and oxyds having double and triple bases. Nature furnishes us with numerous examples of this kind of combinations, by means of which, chiefly, she is enabled to produce a vast variety of compounds from a very limited number of elements, or simple substances. It was long ago well known, that, when muriatic and nitric acids were mixed together, a compound acid was formed, having properties quite distinct from those of either of the acids taken separately. This acid was called _aqua regia_, from its most celebrated property of dissolving gold, called _king of metals_ by the alchymists. Mr Berthollet has distinctly proved that the peculiar properties of this acid arise from the combined action of its two acidifiable bases; and for this reason we have judged it necessary to distinguish it by an appropriate name: That of _nitro-muriatic_ acid appears extremely applicable, from its expressing the nature of the two substances which enter into its composition. This phenomenon of a double base in one acid, which had formerly been observed only in the nitro-muriatic acid, occurs continually in the vegetable kingdom, in which a simple acid, or one possessed of a single acidifiable base, is very rarely found. Almost all the acids procurable from this kingdom have bases composed of charcoal and hydrogen, or of charcoal, hydrogen, and phosphorus, combined with more or less oxygen. All these bases, whether double or triple, are likewise formed into oxyds, having less oxygen than is necessary to give them the properties of acids. The acids and oxyds from the animal kingdom are still more compound, as their bases generally consist of a combination of charcoal, phosphorus, hydrogen, and azote. As it is but of late that I have acquired any clear and distinct notions of these substances, I shall not, in this place, enlarge much upon the subject, which I mean to treat of very fully in some memoirs I am preparing to lay before the Academy. Most of my experiments are already performed; but, to be able to give exact reports of the resulting quantities, it is necessary that they be carefully repeated, and increased in number: Wherefore, I shall only give a short enumeration of the vegetable and animal acids and oxyds, and terminate this article by a few reflections upon the composition of vegetable and animal bodies.