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 prohibited by prudent physicians. Several others, as nitrous acid, lapis infernalis 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, 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 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, which is one and the same thing as weight or gravitation.