436 METALLIC ELEMENTS. 



with oxygen in certain proportions, and combines also with 

 sulphur, chlorine, &c., in the same proportions. Hence, given 

 the formulae of the oxides of a metal, the formulae of its sul- 

 phurets, chlorides, &c., may generally be predicated, as they 

 correspond with the former. Thus the oxides of iron being 

 Fe O and Fe 2 O 3 , the sulphurets are Fe S and Fe 2 S 3 , and the 

 chlorides Fe Cl and Fe 2 C1 3 ; the oxides of arsenic, or arsenious 

 and arsenic acids, being As O 3 and As O 5 , the sulphurets of that 

 metal are As S 3 and As S 5 , and the chlorides As C1 3 and As C1 5 . 

 But sometimes a metal unites with sulphur in more ratios than 

 with oxygen, both iron and arsenic, for example, possessing each 

 a sulphuret to which they have no corresponding oxide, namely 

 martial pyrites and realgar, of which the formulae are Fe S 2 and 

 As S 2 . The potassium family of metals combine also with 

 three and five equivalents of sulphur, without all uniting with 

 oxygen in such high proportions. Again, certain metals of the 

 magnesian and its allied families, such as manganese and chro- 

 mium, form acid compounds with oxygen, to which no corres- 

 ponding sulphurets exist, such as manganic and chromic acids, 

 Mn O 3 and Cr O 3 . But the circumstance that these acids are 

 isomorphous with sulphuric acid, and the metals they contain 

 isomorphous with sulphur, appears to be a sufficient reason 

 why there should not be similar sulphur acids. The chlorides 

 of a metal generally correspond in number, as they always do 

 in composition, with the oxides ; in some cases they are less 

 numerous, but never, I believe, more numerous than the oxides 

 of the same metal. 



Combination takes place within a series, that is, oxides com- 

 bine with oxides, sulphurets with sulphurets. Those members 

 of the same series which differ greatly in chemical characters 

 being most disposed to combine together, as oxygen acids with 

 oxygen bases, sulphur acids with sulphur bases. Chlorides also 

 combine with chlorides, to form double chlorides, and iodides 

 with iodides. 



Compounds belonging to different series, on the contrary, 

 do not combine together, but often mutually decompose each 

 other, when brought into contact. Thus hydrochloric acid and 

 potash do not unite, but form water and chloride of potassium, 

 by mutual decomposition, as explained in the following dia- 

 gram : 



