ix SULPHUR AND PHOSPHORUS 175 



the volume of the air, which ceased after eighteen to twenty 

 days ; the air remaining in the bell-jar was found to have 

 been deprived of its oxygen, since it no longer supported 

 combustion, and in all respects resembled the " atmospheric 

 mofette " or azote. Lavoisier concluded that : 



" The pyrites is a compound of sulphur and iron ; the act of 

 vitriolisation is nothing else than an addition of [oxygen] . . . , 

 an addition which converts the sulphur into vitriolic acid ; 

 but, this acid being in contact with the finely-divided iron, 

 attacks and dissolves it as rapidly as it is formed, and iron 

 vitriol is produced" (Works, II. 211). 



In the case of iron pyrites the action is rendered complex, 

 by the fact that the mineral contains more sulphur than is 

 required to convert the iron into sulphate. But in the case 

 of the other metals, e.g. lead, Berzelius found that the 

 proportions of metal and sulphur in the sulphide are exactly 

 the same as in the sulphate : the change from sulphide to 

 sulphate is then a direct combination of the sulphide with 

 oxygen. 



The converse change of sulphates into sulphides was 

 accomplished by Glauber, who heated the vitriols with 

 charcoal, and thus converted them into " livers " from which 

 part of the sulphur could be precipitated by the addition of 

 acids. 



Sulphuretted hydrogen as an acid, Sulphuretted 

 hydrogen is most conveniently prepared by the action of acids 

 on artificial sulphide of iron (Scheele, Air and Fire, p. 193). 

 Dalton describes an improved method of carrying out this 

 process, as follows : 



" The best way I have found to obtain sulphuretted 

 hydrogen in a pure state, is to heat a piece of iron to a 

 white or welding heat in a smith's forge, then suddenly 

 drawing it from the fire, apply a roll of sulphur ; the two 

 being rubbed together, unite and run down in a liquid form, 

 which soon fixes and becomes brittle. This compound or 



