292 MERCUROTJS CHLORIDE 



in nitric and acetic acids. It is unstable, and on exposure 

 to light or gentle heat decomposes into mercuric oxide and 

 metallic mercury. It is less active than the mercuric oxide, 

 and is frequently used as a stimulant for unhealthy wounds 

 and ulcers, in the form of the black wash (lotio hydrargyri 

 nigra), made by mixing 30 grains calomel, half an ounce of 

 glycerin, 1J ounces of tragacanth mucilage, and sufficient 

 lime-water to make 10 ounces of lotion. 



RED MERCURIC OXIDE. Hydrargyri Oxidum Rubrum. Red 



Precipitate. HgO. 

 YELLOW MERCURIC OXIDE. Hydrargyri Oxidum Flavum. 



Red mercuric oxide is obtained by heating mercurous 

 nitrate until acid vapours cease to be evolved. Yellow 

 mercuric oxide is obtained by the interaction of mercuric 

 chloride and sodium hydroxide (B.P.). The red oxide occurs 

 in orange-red crystals, which become yellow when powdered 

 and brownish-black when heated, recovering, however, their 

 original colour on cooling. It dissolves sparingly in water, 

 but readily in hydrochloric acid. The yellow oxide is con- 

 tained in lotio hydrargyri flava or yellow wash. 



Mercuric oxide is nearly as active and irritant as corrosive 

 sublimate. Two or three grains given to dogs cause fatal 

 gastro-enteritis. Eight to fifteen grains caused colic in 

 horses, and one or two drachms enteritis and death (Hert- 

 wig). It is applied externally as a stimulant and caustic for 

 indolent ulcers, over-luxuriant granulations, chronic scaly 

 eruptions, and glandular enlargements, being used in the 

 form of powder, lotion, or ointment. Red precipitate oint- 

 ment is made with J ounce mercuric oxide and 2J ounces of 

 yellow paraffin ointment. 



MERCUROUS CHLORIDE. Hydrargyri Subchloridum. Calo- 

 mel. Hg 2 Cl 2 . 



Calomel is found native in Carniola and Spain, but in too 

 small amount to be of commercial value. It is obtained by 

 heating a mixture of mercurous sulphate and sodium 



