STATES OF THE GASES. 53 



is formed in this manner, that gas is produced when pyrite is decomposed 

 by steam at high temperatures. 1 



FeS 2 + H 2 O - FeO + H 2 S + S 



As pyrite is frequently present in igneous rocks which generally evolve 

 water-vapor upon the application of heat, the limited quantities of hydro- 

 gen sulphide obtained may be explained in this way. But unless the 

 hydrogen sulphide be removed, this process can proceed only to a certain 

 point, for, according to Berzelius, iron disulphide is formed when FeCO 3 , 

 Fe 3 O 4 , Fe 2 O 3 , or Fe(OH) 3 is heated with hydrogen sulphide to temperatures 

 between 100 and red heat. 2 At red heat, a current of dry hydrogen sul- 

 phide completely converts Fe 3 O 4 into Fe 3 S 4 in two hours, while a still 

 further increase of temperature results in the formation of FeS and a deposit 

 of sulphur. 3 



An inspection of the analyses shows that sulphureted hydrogen is 

 rarely obtained in large amounts from igneous rocks. An average of 75 

 analyses from a wide range of rocks (but omitting bituminous shales) 

 gave 0.59 per cent of this gas. But this figure is not a good working aver- 

 age, since it has been much influenced by the high sulphide percentage 

 of a few individuals. Deducting the five highest of these, the remaining 

 70 analyses give an average of 0.27 per cent of hydrogen sulphide. In 19 

 cases out of the 75, this gas was entirely lacking. 



While it is probable that not much of this gas was given off from the 

 rock material in the first place, a portion of it doubtless disappeared before 

 passing through the pump into the gas-receiver. At the temperature of 

 the combustion-furnace, hydrogen sulphide is apt to be partially dissociated 

 into its elements, thus swelling the already large volume of hydrogen pres- 

 ent. 4 Gautier states that sulphur heated in a tube filled with hydrogen 

 sulphide causes the decomposition of the gas with the result that its sul- 

 phur is added to the free sulphur, while hydrogen, nearly pure, remains. 5 

 When the rock has been considerably weathered, and some of the pyrite 

 oxidized into iron sulphate, so that, in addition to hydrogen sulphide, 

 sulphur dioxide is disengaged, the former gas will be partially or com- 

 pletely decomposed, depending upon the relative proportions of the two 

 gases. 



The bituminous shale from Newsom's Station, near Nashville, Ten- 

 nessee, 6 yielded sulphureted hydrogen to the extent of 30.94 per cent of 

 the total gas, which is equivalent to the unusual amount of 29.38 volumes 

 of hydrogen sulphide from one volume of shale. A specimen of the well- 



1 With an excess of steam the reaction goes further: 3FeS 2 + 4H 2 O = Fe 3 O 4 

 + 3S + H,,. 



2 Berzelius, cited by Graham-Otto, Anorg. Chem., 4, p. 718. 



3 Sidot, Chem. Central-Blatt, vol. 40 (1869), p. 1038. 



4 See p. 47. 



5 Gautier, Comptes Rendus, vol. 132 (1901), p. 189. When pyrite is heated either 

 in a vacuum, or in a stream of dry carbon dioxide, Fe 7 S 8 and free sulphur result (Berzelius, 

 Rammelsberg cited in Gmelin-Kraut, Anorg. Chem., 3, p. 335). 



6 Analysis No. 41. 



