(As. Eq.=75.) Arsenic occurs chiefly in the form of arseniuret of iron, nickel, or cobalt. Metallic arsenic is not employed in medicine; when pure, it is dark steel-coloured, with metallic lustre, crystalline, and brittle; sp. gr. 5.8; very volatile, and when heated gives off an odour like garlic; it forms with oxygen two acids, and combines readily with sulphur.