SULPHATE OF IRON. 129 



sulpliuret of iron ; and from the same reason soils 

 also sometimes contain traces of this salt. 



287. Sulphuret of iron is likewise very commonly 

 found in coal, being sometimes dispersed throughout 

 it in the form of little yellow shining particles, and 

 sometimes as layers or lumps of the solid sulphuret; 

 its presence in coal is for some purposes highly objec- 

 tionable ; because, whether the coal is burnt in its 

 crude state, or after being converted into coke, a 

 quantity of sulphurous acid gas is always formed by 

 the imperfect combustion of the sulphur; and that 

 nauseous-smelling gas causes serious mischief in 

 several operations in the arts, and consequently in 

 such cases, coal free from sulphuret of iron can only 

 be employed. It is the presence of this substance 

 in coal that causes the strong suffocating smell of 

 sulphurous acid sometimes given out by coal and coke 

 fires (173). 



288. The oxides of iron are quite insoluble in 

 water, but many of the salts of iron, like the sulphate, 

 are readily soluble in it ; the solutions of these salts 

 are all decomposed when alkali is added to them ; 

 this combines with the acid, and the oxide of iron is 

 separated as an insoluble powder. 



289. The most important of the salts of iron is 

 the sulphate, or common green vitriol : it may be 

 formed by acting upon iron by dilute sulphuric acid ; 

 when this is done, a large quantity of hydrogen gas 

 is given off, in consequence of the decomposition of 

 a portion of water; the oxygen combines with the 



