228 On the Oxidation of Metals in general, 



compositions ; it may be transformed also into acidulated 

 sulphate by heating it with iron ; this is what is done in se- 

 veral vitriol manufactories, and particularly at Beauvais, 

 where this salt is extracted from pyritous turf, which is 

 burned, and which without that would yield acid sulphate, 

 less beneficial in commerce. This may be done with every 

 kind of acid sulphate, whatever is its origin ; whether na- 

 tural or artificial, the transformation always takes place : it 

 would still be made whether the oxide of iron was green or 

 red. The acidulated and acid sulphates of white iron are 

 both precipitated white by the alkalies ; they instantly de- 

 compo;5e oxygenated muriatic acid, and pass, in proportion 

 as more or less of it is introduced into them, to that state of 

 sulphate in which the oxide is green or red ; it is in this 

 manner that the oxygenated muriatic acid acts upon the pure 

 white oxide ; such also is the action of the air upon the com- 

 binations of this oxide with acids, and particularly with sul- 

 phuric acid. This is the reason why the colour of these so- 

 lutions is not constant ; from the green they pass to the red ; 

 the liquor is troubled, deposits a yellow matter, and then 

 ceases to be coloured. All these phaenomena may be ex- 

 plained from natural causes, and are the consequences of 

 the properties which the other sulphates of iron present to 

 us, of which we are now about to speak. 



The acidulated and acid sulphates of green iron, both of 

 which result from the combination of the green oxide of 

 iron with sulphuric acid, present the most striking differ- 

 ences. The first (being that which is only a little acid) 

 does not crystallize at all, not being able to exist except in 

 a liquid state ; if submitted to evaporation it absorbs oxygen 

 from the air, becomes troubled, and deposits neutral sul- 

 • phate, yellow, insoluble, and very much oxidated : thus it 

 transforms itself into acid sulphate, in which the oxide is 

 always green, but itself almost colourless, and which resists 

 every kind of decomposition much more than the other: 

 although it has green oxide for its base, it is red itself: this 

 is what occasions the error of most chemists, who, down to 

 the present moment, have regarded it as a sulphate very 

 much oxygenated : thence it again happens that the acidu- 

 lated 



