Metallic arsenic has an iron-gray colour, a specific gravity of 8·308, and a crystalline fracture. It is very brittle. It has a strong tendency to oxidate, so that it undergoes this change in air, in water, and even in alcohol. In air, particularly when moist, it becomes rapidly tarnished, a black powder being formed, which some have regarded as a regular protoxide.[488]—When exposed to heat, metallic arsenic is usually said to sublime at the temperature of 356° F.; but according to some late experiments by Dr. Mitchell of Philadelphia this does not happen under a low red heat, luminous in the dark.[489] In close vessels it condenses unchanged; but when heated in the open air, it passes to the state of white oxide, and rises in white fumes. This substance is a sesquioxide, consisting of two equivalents of metal and three of oxygen. Another oxide likewise exists, which contains two equivalents of metal and five of oxygen, and, possessing strong acid properties, is denominated arsenic acid. The sesquioxide and arsenic acid unite with bases, and produce compounds which, with the exception of those they form with the alkalis, are mostly insoluble. Metallic arsenic unites with sulphur in two proportions, forming an orange-red and a sulphur-yellow compound. The compounds of arsenic have very little chemical action with vegetable and animal principles. Of the compounds which arsenic thus forms, those which it will be necessary to particularize are the following:—1. The protoxide of Berzelius, or _fly-powder_. 2. The arsenious acid, or _white arsenic_. 3. The arsenite of copper, or _mineral green_. 4. The arsenite of potass as contained in _Fowler’s solution_. 5. The arsenite of potass; 6. The various sulphurets, pure and impure, namely, _realgar_, _orpiment_, and _king’s yellow_; and 7. Arseniuretted-hydrogen gas. _Of the Tests for Fly-powder._ This substance is rarely known as a poison in Britain, but is a familiar poison in France and Germany, under the names of _Poudre à mouches_, and _Fliegenstein_. Of late it has been occasionally used in Scotland for poisoning rats. It is a fine grayish-black powder, formed by exposing powdered arsenic for a long time to the air; but it also frequently contains fragments of the metal. It is usually considered by chemists to be a mixture of metallic arsenic and its white oxide. It is acted on by water, the white oxide being found ere long in solution by its proper tests. Oxidation and solution, however, are also effected upon pure metallic arsenic in the same manner. A thousand grains of water take up a grain in the course of half an hour when boiled on the metal.[490]