a quality belonging to several substances, by the acrimony of which the parts of living animals may be corroded and destroyed. Bodies which have this quality, when taken internally, are true poisons. The causticity of some of these, as of arsenic, is so deadly, that even their external use is proscribed by prudent physicians. Several others, as nitrous acid, lapis internalis or lunar caustic, common caustic, butter of antimony, are daily and successfully used to consume fungous flesh, to open ulcers, &c. They succeed very well when properly employed and skilfully managed.

The causticity of bodies depends entirely on the state of the saline, and chiefly of the acid, matters they contain. When these acids happen to be at the same time much concentrated, and slightly attached to the matters with which they are combined, they are then capable of acting, and are corrosive or caustic. Thus fixed and volatile alkalies, although they are themselves caustic, become much more so by being treated with quicklime; because this substance deprives them of much fat and inflammable matter, and all their fixed air, which binds and restrains the action of their saline principle. By this treatment, then, the saline principle is more disengaged, and rendered more capable of action. Also all combinations of metallic matters with acids form salts more or less corrosive, because these acids are deprived of all their superabundant water, and are besides but imperfectly saturated with the metallic matters. Nevertheless, some other circumstance is necessary to constitute the causticity of these saline metalline Cauticity, metallic matters. For the same quantity of marine acid, which, when pure and diluted with a certain quantity of water, would be productive of no harm, shall, however, produce all the effects of a corrosive poison, when it is united with mercury in corrosive sublimate, although the sublimate shall be dissolved in so much water that its causticity cannot be attributed to the concentration of its acid. This effect is, by some chemists, attributed to the great weight of the metallic matters with which the acid is united; and this opinion is very probable, seeing its causticity is nothing but its dissolving power, or its disposition to combine with other bodies; and this disposition is nothing else than attraction.

On this subject Dr Black observes, that the compounds produced by the union of the metals with acids are in general corrosive. Many of them applied to the skin destroy it almost as fast as the mineral acids; and some of the most powerful potential cauteries are made in this way. Some are reckoned more acid than the pure acids themselves; and they have more powerful effects when taken internally, or at least seem to have. Thus we can take 10 or 12 drops of a fossil acid, diluted with water, without being disturbed by it; but the same quantity of acid previously combined with silver, quicksilver, copper, or regulus of antimony, will throw the body into violent disorders, or even prove a poison, if taken all at once.

This increased activity was, by the mechanical philosophers, supposed to arise from the weight of the metallic particles. They imagined that the acid was composed of minute particles of the shape of needles or wedges; by which means they were capable of entering the pores of other bodies, separating their atoms from each other, and thus dissolving them. To these acid spicules the metallic particles gave more force; and the momentum of each particular needle or wedge was increased in proportion to its increase of gravity by the additional weight of the metallic particle. But this theory is entirely fanciful, and does not correspond with facts. The activity of the compound is not in proportion to the weight of the metal; nor are the compounds always possessed of any degree of acrimony; neither is it true that any of them have a greater power of destroying animal substances than the pure acids have.

There is a material difference between the powers called stimulants and corrosives. Let a person apply to any part of the skin a small quantity of lunar caustic, and likewise a drop of strong nitrous acid, and he will find that the acid acts with more violence than the caustic; and the disorders that are occasioned by the compounds of metals and acids do not proceed from a causticity in them, but from the metal affecting and proving a stimulus to the nerves; and that this is the case, appears from their affecting some particular nerves of the body. Thus the compounds of regulus of antimony and mercury with the vegetable acids, do not show the smallest degree of acrimony; but, taken internally, they produce violent convulsive motions over the whole body, which are occasioned by the metallic matter having a power of producing this effect; and the acid is only the means of bringing it into a dissolved state, and making it capable of acting on the nervous system. In general, however, the compounds of metallic substances with acids may be considered as milder than the acids in a separate state; but the acid is not so much neutralized as in other compounds, for it is less powerfully attracted by the metal; so that alkaline salts, absorbent earths, or even heat alone, will decompose them; and some of the inflammable substances, as spirit of wine, aromatic oils, &c., will attract the acid, and precipitate the metal in its metallic form; and the metals can be employed to precipitate one another in their metallic form; so that the cohesion of these compounds is much weaker than those formed of the same acids with alkaline salts or earths.