174 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



in 1811-1812 (Ostwald's Klassiker, XXXV. 31-34), to 

 determine the proportions of iron and of sulphur in natural 

 iron pyrites, in order to compare it with the artificial 

 sulphide of iron prepared by heating iron with sulphur. 

 He found that : 



9*93 grams of iron pyrites x gave 6'6o grams of oxide, 

 whilst, by an indirect method, 



2 grams of artificial sulphide gave 1-82 grams of oxide, 



the oxide containing 69^34% of iron. The composition of 

 these two substances was therefore : 



Pyrites Iron 46*08 Sulphur 53*92% 



Artificial sulphide 63 37% 



Conversion of sulphides to sulphates, The sulphides 

 also oxidise slowly when exposed to air and water, but in 

 this case the product is usually a sulphate, i.e., a compound 

 of the calx or oxide of the metal with sulphuric anhydride. 

 On account of the readiness with which they unite with 

 oxygen, the solutions prepared by dissolving sulphur in potash, 

 soda, lime and baryta, and mixtures of iron filings with sulphur, 

 were used by Hales, Priestley, Scheele and others for 

 absorbing the active part of the air, as well as for 

 "diminishing" nitrous air, i.e., converting nitric into nitrous 

 oxide. It was not, however, until Lavoisier had developed 

 his "oxygen" theory of combustion that these changes 

 were satisfactorily explained. In a paper "On the Vitriol- 

 isation of Pyrites," published in 1777 ( Works, II. 209), 

 Lavoisier showed that the formation of green vitriol only 

 takes place in presence of air and can be stopped by covering 

 the pyrites with oil, or by keeping it under water. On 

 exposing the pyrites in a bell-jar of air over water, he found 

 that the vitriolisation was accompanied by a diminution in 

 1 10 grams of pyrites containing 0*07 gram of silica. 



