[oxide of arsenic, white oxide of arsenic, arsenious acid.] Metallic arsenic combines naturally with oxygen and sulphur. "We have two natural compounds of arsenic with oxygen, viz., arsenious acid and arsenic acid ; the former is the preparation of arsenic in common use among physicians of our school ; the latter is met with in combination with bases, such as arseniate of silver, soda, iron, lime, etc. Orpiment and realgar are the compounds of arsenic with sulphur. Both are used in the arts and were well known to the ancients. Pereira gives an accurate description of the process of obtaining ARSENICUM ALBUM. 447 arsenious acid on a large scale from arsenical iron. This process is resorted to at Altenburg, in Silesia, where this ore is obtained. It consists of a series of very simple operations :