212 SULPHURETS, AND SULPHATES OF IRON. 



prevented as much as possible ; and it may occasionally be summer- 

 fallowed with advantage, in order also that the per-oxide may absorb 

 from the air those volatile substances which are likely to prove benefi- 

 cial to the growth of the future crops. 



2°. Sulpfiurets of Iron. — Iron occurs in nature combined with sulphur 

 in two proportions, forming a sulphuret and a fez-sulphuret. These 



consist respectively — 



Iron. Sulphur. Symbol. 



The sulphuret . . . 62-77 37-23 Fe S 

 The bi-sulphuret of . 45-74 54-26 Fe 83 

 and are both tasteless and insoluble in water. 



1°. The first of these, the sulphuret (Fe S), occurs occasionally in 

 boggy and marshy soils, in which salts of iron exist, or into which they 

 are carried by rains or springs. It is notitself directly injurious to vege- 

 tation, but when exposed to the air it absorbs oxygen and forms sulphate 

 of iron, which, when present in sufficient (]uantity, is eminently so.* 



2°. The bi-sulphuret, or common iron pyrites (Fe So), is exceedingly 

 abundant in nature. It occurs in nearly all rocky formations — and in 

 most soils. It abounds in coal, and is the source of the sulphurous smell 

 which many varieties emit while burning. It generally presents itself 

 in masses of a yellow colour and metallic lustre, more or less perfectly 

 crystallized in cubical forms, so brittle and hard as to strike fire with 

 steel, and of a specific gravhy four and a half times greater than that of 

 water (Sp. gr. 4, 5). When heated in close vessels it parts with nearly 

 one-half of its sulphur, and hence is often distilled lor the sulphur it 

 yields. 



In the air it absorbs oxygen, in some cases — as in the waste coal 

 heaps — with such rapidity as to heat, take fire, and burn. By this ab- 

 sorption of oxygen (oxidation), sulphuric acid and sulphate of iron are 

 produced." In the alum shales the iron pyrites abounds, and these are 

 often burned for the purpose of converting the sulphur and sulphuric 

 acid for the subsequent manufacture of alum. 



3°. Sulphates of Iron. — Of the sulphates of iron which are known, 

 there is only one — the common green vitriol o^ ihe shops — that occurs in 

 the soil in any considerable quantity. There are few soils, perhaps, in 

 which its presence may not be detected, though it is in bogs and marshy 

 places that it is most generally and most abundantly met with. It is 

 often exceedingly injurious to vegetation in such localities, but it is de- 

 composed by quick-lime, by chalk, and by all varieties of marl, and 

 thus its noxious effects may in gereral be entirely prevented. To soils 

 which abound in lime, it may even be applied with a beneficial effect. 



When a solution of this salt is exposed to the air it speedily becomes 

 covered with a pellicle of a yellow ochrey colour, which afterwards falls 

 as a yellow sediment. This sediment consists of per-oxide of iron, con- 

 taining a little sulphuric acid ; but by the separation of this oxide the 

 sulphuric acid is left in excess in the solution, which becomes sour, and 



* Yet in small quantity it may be beneficial. Thus Sprengel mentions that the subsoil of 

 a moor near Hanover, which contains some of this sulphuret of iron, produces astonishing 

 effects when laid as a top-dressing on the grass lands. Tlie explanation of this is, that ihe 

 pyrites absorb oxygen and is converted into sulptiale, and thus re-produces the remarkable 

 effects observed on the additijnof gypsum, of sulphuric acid, or of sulphate of soda, to simi- 

 lar grass lands. 



