182 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



" Phosphoretted hydrogen has then no acid properties ; 

 and this is its principal difference from sulphuretted hydro- 

 gen. It follows : (i) that the gas is liberated as fast as it is 

 formed, whilst sulphuretted hydrogen is held in combination 

 with the alkali and the water ; (2) that the alkaline phos- 

 phides are decomposed in contact with water" (Ann. de 

 Chimie, 1798, 25. 267). 



SUMMARY AND SUPPLEMENT. 

 A. AND B. SULPHUR. 



Sulphur or brimstone is found in the free state and also in 

 combination with metals in the mineral sulphides, such as : 

 Iron pyrites FeS 2 (persulphide of iron) 

 Copper pyrites FeCuS 2 (sulphide of iron and copper) 

 Cinnabar HgS (sulphide of mercury) 



Galena PbS (sulphide of lead) 



Blende ZnS (sulphide of zinc). 



A part of the sulphur in iron pyrites can be distilled out : 



3FeS 2 -> Fe 3 S 4 + S 2 . 



The whole of the sulphur in the mineral sulphides can, however, 

 be removed by burning, whereby both metal and sulphur are 

 converted into oxide, e.g. : 



3FeS 2 + 8O 2 -> Fe 3 O 4 + 6SO 2 

 2ZnS + 3O 2 -> 2ZnO + 2SO 2 . 



Sulphur burns in air, without producing any change of volume, 

 to a pungent gas, sulphurous anhydride (sulphur dioxide, SO 2 ) : 



S + O 2 -> SO 2 . 

 i vol. i vol. 



The gas dissolves freely in water, forming a weak volatile acid, 

 sulphurous acid, H 2 SO 3 , but is expelled by boiling and by 

 exposure to air. The acid can be fixed by means of alkalis 

 or bases, with which it combines to form salts known as 

 sulphites, e.g. : 



Sulphite of soda or sodium sulphite NagSOg 

 Sulphite of potash or potassium sulphite K 2 SO 3 

 Sulphite of lime or calcium sulphite CaSO 3 . 



