"At the time Cavendish made his study of compounds, chemists still had not been able to "determine what it /arsenic/ really is, or to what class of bodies it belongs." ...[A]rsenic behaves as a in some states, and like a salt in other states. ...[L]ike every metallic , arsenic [could be] changed into a metallic form... "regulus of arsenic"... [by combining] with "phlogiston." ...[L]ike salts, arsenic is soluble in water. ...[N]either ic nor ne, yet Macquer claimed, behaving as if it were an acid. ...In other ways ...arsenic differs from other... calces: it is volatile with a strong smell, it is fusible, it unites with metals and s—the difference that Macquer and Cavendish picked up on—it decomposes nitre when distilled with it. From the standpoint of affinities... arsenic is exceptional too."