226 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



the compounds of chlorine with lead, silver, mercury, 

 potassium and sodium might " be considered as a class of 

 bodies related more to the oxides " than to the ordinary 

 salts (A.C.R. IX. 34)." Gay-Lussac recognised a still 

 closer analogy between chlorine and sulphur, and between 

 the chlorides and sulphides. According to these views a 

 true analogy is supplied by comparing the muriates or 

 chlorides with binary compounds, such as : 



LITHARGE (OXIDE OF LEAD) = {oXYGEN 



GALENA (SULPHIDE OF LEAD) = {SULPHUR 



and not with ternary compounds, such as : 



("LITHARGE = ( LEAD 



SULPHATE OF LEAD = J VOXYGEN 



[SULPHURIC ANHYDRIDE = | SULPHUR 



Displacement of sulphur and oxygen by chlorine. The 



analogy between chlorine, sulphur, and oxygen is shown by 

 the readiness with which these elements can be interchanged 

 in their compounds with the metals. Thus Scheele, in his 

 first description of chlorine, stated that by contact with the 

 gas " cinnabar became white on the surface, and when the 

 piece was washed in water a pure sublimate solution was 

 obtained, but the sulphur was not altered" (A.C.R. XIII. 

 9). This action may be represented by the equation : 

 Cinnabar + chlorine -> corrosive sublimate + sulphur 



(Mercury-|- (Mercury+chlorine) 



sulphur) 



A similar process is now used on a large scale for con- 

 verting sulphide ores of zinc, lead, and silver into chlorides. 

 The conversion of oxides into chlorides was described 

 by Davy, who thus obtained a direct proof of the presence 

 of oxygen in the earths. Davy found that : 



" When baryta, strontia, or lime, is heated in [chlorine] to 



