Synthetic Dyes

DYES, SYNTHETIC).

Sodium arsenate, used in calico printing, is prepared by adding sodium carbonate to aqueous arsenic acid or by heating sodium arsenite with sodium nitrate. This salt, which crystallizes as Na2HAs04, I is isomorphous with the corresponding phos phate (see CRYSTALLOGRAPHY.) Soluble arsenates and phosphates have many properties in com mon; they both yield colourless crystalline precipitates with am moniacal solutions of ammonium and magnesium chlorides, and with ammonium molybdate they furnish similar yellow precipi tates (arsenomolybdate and phosphomolybdate), but with silver nitrate arsenates give a reddish-brown precipitate, whilst from phosphates a yellow precipitate results.

Sulphides of Arsenic.

Three sulphides are known, two occur ring native, yellow orpiment (auripigmentum), in yellow prisms and realgar, in reddish orange prisms, whereas the pentasulphide is produced artificially either by heating together the trisulphide and sulphur or by acidifying a cold solution of thioarsenate. The tri- and penta-sulphides are insoluble in water but dissolve in alkali sulphides to give alkali thioarsenites and thioarsenates respectively.

Realgar and orpiment are used as pigments and in pyrotechny; they have been produced artificially for these purposes by heating white arsenic with appropriate amounts of sulphur.

Pharmacology and Therapeutics.

Of arsenic and its corn pounds, arsenious oxide and Fowler's solution (potassium arse nite) are in very common use. The iodide of arsenic is one of the ingredients of Donovan's solution (see MERCURY) ; and iron ar senate, or a mixture of ferrous and ferric arsenates with some iron oxide, is of great use in certain cases. An iron and arsenic injec tion is widely employed in anaemic conditions and constitutes one of the few treatments which are effective in such circumstances. Sodium arsenate is somewhat less commonly prescribed.

Externally, arsenious oxide 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 gas tric 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 cor puscles 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, etc., but it is also stored up in the body, mainly in the liver and kidneys. Arsenic is administered on ac count of its tonic effects on the general and nervous systems. It is also considered by many authorities to have an antiperiodic action as in malaria and it is known to be effective in various chronic skin affections.

Externally arsenious oxide has been much used by quack doc tors to destroy morbid growths, etc., a paste or solution being sup plied, strong enough to kill the mass of tissue, thus making it slough out quickly. But accidents have resulted from the arsenic being absorbed, the patient being thereby poisoned. Internally it is useful in certain forms of dyspepsia, but as some patients are quite unable to tolerate the drug, it must always be administered in very small doses at first, the quantity being slowly increased as tolerance is shown. Children as a rule bear it better than adults. It should never be given on an empty stomach, but always after a full meal. It is the routine treatment for pernicious anaemia and Hodgkin's disease, though here again the drug may be of no avail. For the neuralgia and anaemia following malaria, for rheu matoid arthritis, for chorea and also asthma and hay fever, it is constantly prescribed with excellent results. Certain skin diseases, as psoriasis, pemphigus and occasionally chronic eczema are much benefited by its use, though sometimes a too prolonged course will produce the very lesion for which in other circumstances it is a cure. Occasionally, as among the Styrians, individuals acquire the habit of arsenic-eating, which is said to increase their weight, strength and appetite, and to clear their complexion. The probable explanation is that an antitoxin is developed within them, but definite proof is not at present forthcoming. Arsenic-eating is also found in Salzburg and the Tirol.