_Pharmacology._--Of arsenic and its compounds, arsenious acid (dose 1/60-1/15 gr.) and its preparation liquor arsenicalis, Fowler's solution (dose 2-8 [minim]), are in very common use. The iodide of arsenic (dose 1/20-1/5 gr.) is one of the ingredients of Donovan's solution (see MERCURY); and iron arsenate (dose 1/16-1/4 gr. in a pill), a mixture of ferrous and ferric arsenates with some iron oxide, is of great use in certain cases. Sodium arsenate (1/40-1/10 gr.) is somewhat less commonly prescribed, though all the compounds of this metal have great value in experienced hands. Externally, arsenious acid is a powerful caustic when applied to raw surfaces, though it has no action on the unbroken skin. Internally, unless the dose be extremely small, all preparations are severe gastro-intestinal irritants. This effect is the same however the drug be administered, as, even after subcutaneous injection, the arsenic is excreted into the stomach after absorption, and thus sets up gastritis in its passage through the mucous membrane. In minute doses it is a gastric stimulant, promoting the flow of gastric juice. It is quickly absorbed into the blood, where its presence can be demonstrated especially in the white blood corpuscles. In certain forms of anaemia it increases the number of the red corpuscles and also their haemoglobin content. None of these known effects of arsenic is sufficient to account for the profound change that a course of the drug will often produce in the condition of a patient. It has some power of affecting the general metabolism, but no wholly satisfactory explanation is forthcoming. According to Binz and Schultz its power is due to the fact that it is an oxygen-carrier, arsenious acid withdrawing oxygen from the protoplasm to form arsenic acid, which subsequently yields up its oxygen again. It is thus vaguely called an alterative, since the patient recovers under its use. It is eliminated chiefly by the urine, and to a less extent by the alimentary canal, sweat, saliva, bile, milk, tears, hair, &c., but it is also stored up in the body mainly in the liver and kidneys.