MERCURY. 321 



porrigo, and other cutaneous diseases, in doses of from gr. v. to 3SS. 

 twice a-day, blended with any viscid substance. The Unguentum 

 Hydrargyri, in which the metal is in the state of protoxide, is the 

 form under which mercury is introduced into the system, through 

 the superficial absorbents. From one to two drachms of the strong 

 ointment is forced through the cuticle, by friction upon the inside 

 of the thighs and calves of the legs every night in a warm room. 

 or before the fire, until the constitution is affected. This 

 mode of applying mercury is frequently resorted to in venereal 

 cases, in obstinate hepatic obstructions, and in some dropsical 

 affections, particularly hydrops articuli, and hydrocephalus, where 

 the object of the treatment is to stimulate the absorbents. Mercu- 

 rial plaster is the metal triturated with oil and resin, and mixed 

 with litharge plaster ; it is sometimes applied to indolent glandular 

 tumours and indurations, as a discutient. 



Red Oxide, or Peroxide of Mercury. — By exposing mercury 

 to heat in a shallow vessel, at a temperature of about GOO , it slowly 

 absorbs oxygen from the air, and is converted into a peroxide. It 

 is obtained in the form of scales and crystaline grains of a bright 

 red colour ; it is the Oxydum hydrargyri ruhrwm of the London 

 Pharmacopoeia; and was formerly called Hydrargyrum precipi- 

 tatum per se, and hydrargyrum calcinatum. The red oxide is a 

 very active and dangerous preparation of mercury, and is now very 

 seldom exhibited internally, except where other remedies fail. 

 The best form for its administration is that of pill, combined with 

 opium in very small doses, such as one-fourth or one-eighth of a 

 grain, night and morning. It is principally used as an external 

 stimulant and escharotic. For this purpose it is either sprinkled 

 upon the diseased surface, finely levigated, or applied in the form 

 of ointment, mixed with lard. The ointment is one of the most 

 common stimulating dressings for various kinds of ulcers, and is 

 an excellent application to the inside of the eye-lids in cases of 

 chronic ophthalmia, and opacities of the cornea. The peroxide, 

 prepared by dissolving the metal in nitric acid and then evaporating 

 the solution to dryness, is the hydrargyri nitrico -Oxydum and is 

 more acrid and caustic than the oxide obtained by heat. The 

 white precipitate of mercury, Hydrargyrum prccipitalum album, 



