SULPHIDE, or YDROSULP IICRIC ACID, IT„S. A gaseous acid compound of sulphur and hydrogen, known since the sixteenth century, hut first care fully investigated by Scheele in 1777. It occurs uncombined in certain mineral waters and is formed in the decomposition of albuminous. sub stances containing sulphur. It also occurs as a product of transformation of gypsum (calcium sulphate) and other metallic sulphates and is a constituent of volcanic exhalations. Rut what ever its origin, it does not long remain un changed in the air, being oxidized with the great est ease. The usual method of preparing the gas consists in causing sulphuric or preferably hy drochloric acid to act on ferrous sulphide. The following rule usually holds good: Sulphureted hydrogen is formed by the action of acids on the sulphides of those metals (e.g. iron, zinc, man ganese, calcium, magnesium, sodium, potassium, etc.) which can directly decompose the acids with liberation of hydrogen. Sulphureted hy drogen is a colorless gas, somewhat heavier than air, soluble in water and more so in alcohol. the solutions reddening blue litmus. paper. :Mixed
with 13/4 volumes of oxygen and ignited. it ex plodes. the products of the reaction being water vapor and sulphur dioxide (S0,1. Under the influence of cold and pressure. sulphureted hy drogen condenses to a colorless mobile liquid. Sulphureted hydrogen is very poisonous. and even small proportions of it in the air are dan gerous. The presence of free sulphureted hydro gen may be demonstrated by means of a strip of filter-paper soaked with a solution of lead acetate, the paper turning brown or black, owing, to the formation of lead sulphide. Chlorine and iodine decompose sulphureted hydrogen. and this is why the odor of the latter is readily de stroyed by bleaching-powder. Sulphureted hy drogen is used in the manufacture of certain metallic sulphides, for the purification of sul phuric acid. and extensively in analytical chem istry. The sulphureted hydrogen produced in certain industrial processes is utilized by burn it and transforming the resulting sulphur di oxide either into sulphuric acid or into free sul phur.