36 



which has been dried at 400 or 500 Fahr., and is kept between 

 400 and 600 Fahr. during the passage of the gas and vapour, not 

 a trace of bisulphide of carbon passes from the tube, but sulphide of 

 hydrogen does pass. The lime is darkened by a deposit of carbon, 

 and a sublimate of sulphur is found in the exit-tube. A considerable 

 excess of hydrogen should be used, or else a portion of the bisul- 

 phide of carbon vapour is carried over by the current and escapes 

 decomposition. That this reaction is not the result of heat merely, 

 but is a truly chemical one which the base under the influence of 

 heat effects, and the remarkable fact that slaked lime when heated 

 forms, but does not unite with, sulphide of hydrogen, receive illus- 

 tration from the following experiments. 



Hydrogen and vapour of sulphide of carbon were passed through 

 1st, cold slaked lime ; 2nd, cold clay ; 3rd, hot oxide of iron used 

 at a gas-works in purification ; 4th, hot broken bricks ; 5th, hot 

 broken glass, without in any instance producing sulphide of hydrogen. 

 On the contrary, when passed through (1) hot lime and (2) hot 

 clay, sulphide of hydrogen was formed and passed over immediately, 

 and continued to pass as long as the current was kept up. The lime, 

 when cooled out of contact with the air, gave no sulphide of hydro- 

 gen upon being supersaturated with dilute sulphuric acid, but clay 

 when thus treated gave off much. 



The hydrogen used was in all cases passed through lime and over 

 lead-paper, to secure its being free from sulphide before use. On 

 one occasion, when the clay had been imperfectly dried before heating, 

 I observed much sulphurous acid instead of sulphide of hydrogen. 

 I therefore passed hydrogen, bisulphide of carbon vapour, and steam 

 over hot clay which had been properly dried. At first sulphide of 

 hydrogen passed over alone, then mixed with sulphurous acid, which 

 at length passed alone. Subsequently sulphide of hydrogen passed, 

 and at length sulphurous acid ceased. As the one gas increased the 

 other diminished, and throughout the experiment sulphide of car- 

 bon vapour passed undecomposed. It is shown by this experiment 

 that hot clay in the presence of more water than forms a hydrate 

 acts very differently from the same clay when dry, and the whole 

 subject deserves a full investigation. 



