404 COMPOUNDS OF HYDROGEN. 



of the gas in that manner, and becomes sulphuret of potassium, 

 which unites with the other moiety without decomposition, 

 forming a hydrosulphuret of the sulphuret of potassium. The 

 action of other alkaline metals upon sulphuretted hydrogen is 

 similar. 



This compound has a weak acid reaction, and is generally 

 classed with the hydrogen acids. It does not combine and 

 form salts with basic oxides, but it unites with basic sulphurets, 

 such as sulphuret of potassium, and forms compounds which 

 are strictly comparable with hydrated oxides. When sulphu- 

 retted hydrogen is passed over lime at a red heat, both com- 

 pounds are decomposed, and water with sulphuret of calcium is 

 formed. The oxides of nearly all the metallic salts, whether 

 dry or in a state of solution, are decomposed by sulphuretted 

 hydrogen in a similar manner. But in those salts, of which the 

 metallic sulphurets are dissolved by acids, such as salts of iron, 

 zinc and manganese, a small quantity of a strong acid entirely 

 prevents precipitation. These sulphurets are generally coloured, 

 and many of them are black ; hence, the effect of sulphuretted 

 hydrogen in blackening salts of lead and silver, which renders 

 these compounds so sensitive as tests of the presence of that 

 substance. Sulphuretted hydrogen also tarnishes certain metals, 

 such as gold, silver and brass, so that utensils of which these 

 metals are the basis should not be exposed to this gas. 



Persulphuret of hydrogen. When carbonate of potash is 

 fused with half its weight of sulphur, a sulphuret of potassium 

 is formed containing a large excess of sulphur, which affords a 

 solution in water of an orange red colour. The protosulphuret 

 of potassium, with hydrochloric acid, gives sulphuretted hydro- 

 gen and chloride of potassium : H Cl and K S = H S and K Cl. 

 But when the red solution of persulphuret of potassium is 

 poured in a small stream, into hydrochloric acid, diluted with 

 two or three volumes of water, while chloride of potassium is 

 formed as before, the sulphuretted hydrogen produced combines 

 with the excess of sulphur present, and forms a yellowish oily 

 fluid, the persulphuret of hydrogen, which falls to the bottom of 

 the acid liquor. The result of the combination in this case 

 appears rather capricious ; for if the acid and persulphuret of 

 potassium be mixed in the other way, if the acid be added 

 drop by drop to the alkaline sulphuret, then sulphuretted hy- 

 drogen gas is evolved, the whole excess of sulphur precipitates, 



