Sulphydric Acid, Or Sulphuretted Hydrogen Hydrosilphuric Acid, a gaseous compound first examined by Scheele in 1777; symbol, H2S; chemical equivalent, 34. It consists of two volumes of hydrogen and one of sulphur vapor condensed into two volumes, which form its combining measure. Its density is 1191.2, air being 1000. It is a colorless gas, has a slight acid reaction, and a most offensive odor, recognized in rotten eggs, dock mud, cesspools, many mineral waters, and putrefying organic matters containing sulphur. It extinguishes flame, but burns itself in contact with air with a blue flame, depositing sulphur. It is condensed by a pressure of 17 atmospheres at 50° into a colorless liquid, and was solidified by Faraday by cooling to - 122° into a white crystalline translucent substance. Water absorbs 2 1/2 times its volume of the gas; alcohol 6 volumes. It blackens the salts of lead and of many other metals, forming sulphides of the metals. These being insoluble and made readily visible by their peculiar colors, even in minute quantity, the acid is a convenient test for determining the presence of the metals in solutions, and distinguishing them by the color of the precipitate and it3 other properties. Its aqueous solution and its solution in ammonia (hydrosulphide of ammonium) are among the useful chemical reagents.

The gas is exceedingly noxious to inhale. Thenard found that a small bird would die in air containing 1/1500 part of it, and a horse in air that contained 1/250 of it. The gas is neutralized and decomposed by chlorine and iodine, which unite with its hydrogen; and the former, furnished by chloride of lime wet with strong vinegar, is a convenient antidote and disinfectant of the gas. Nitrate of lead, chloride of zinc, sulphate of iron, and sulphate of manganese are also efficacious in this respect. The presence of the gas is detected by its odor, and by its blackening a paper wet with a solution of acetate of lead. It is the cause of the discoloration of white lead paint in the apartments of houses, also of the blackening of silver spoons when these are used with boiled eggs, the albumen of the white of the egg furnishing the sulphur for the production of the gas. - To prepare hydrosulphuric acid, the ingredients employed are a ferrous sulphide, made by exposing to a low red heat 4 parts of coarse sulphur and 7 of iron filings, and diluted sulphuric acid. By pouring the acid upon broken lumps of the compound in a gas bottle, the gas is evolved, and may be collected in a bell glass over water at 80° or 90°, or over brine. It is absorbed by cold water.

It may also be obtained by the action of hydrochloric acid upon antimo-nious sulphide. The reactions in each case are thus expressed: FeS + H2SO4 = FeSO4 + H2S. Sb2S3 + (HCl)a = (SbCl3)2 + (H2S)s.