The iodide of arsenic was introduced into our national Pharmacopoeia for the preparation of the solution of iodide of arsenic and mercury described below. it is made by first rubbing together one part of metallic arsenic and five parts of iodine, and then heating the mixture very gently till it melts. it is an orange-red, crystalline solid, volatilizable by heat, and wholly soluble in water. in composition, it is thought to be a teriodide, consisting of one eq. of arsenic and three eqs. of iodine. The use chiefly made of it therapeutically has been by Biett as an external application in corroding tubercular diseases of the skin, in which he employed an ointment composed of three grains of the iodide and an ounce of lard. it has been given internally also in skin diseases, and ulcers either cancerous, or resembling cancer. The dose is one-eighth of a grain, in pill or solution, three times a day.