274 OXTDATTONS OF IROW. 



to dissolve a given quantity of iron in muriat 



and, 



after having precipitated it by an alkali, to dry it in an ex r 

 hausted receiver by means of a leas. 



f^xigenation 

 pot the cause 

 of the differ- 

 ence of the 

 salt?. 



Mistake of 

 Seheele. 



The air in the 

 water has no 

 effect, 



Influence of the air on solutions of iron. 



All the false notions diffused through the pneumatic 

 theory of iron arise from having ascribed the colour and 

 other properties, that distinguish the green and red salts, to 

 a difference of oxigenation. Tins difference once esta- 

 blished as a principle, nothing was more natural, than to 

 refer to the same cause the transition of the salts of iron 

 from green to red, on exposure to the air; particularly as 

 the circumstances, that sometimes accompany this pheno- 

 menon, readily agree with this explanation. 



The authority of Seheele gave additional weight to thi§ 

 illusion. This celebrated chemist had observed, that, on dis- 

 solving green sulphate of iron in water, a sediment of green 

 oxide commonly remained ; and hence he inferred, that it 

 was owing to the air contained in tne water, which super- 

 oxided part of the green oxide ; and this, becoming red, in- 

 creased its saturating power with respect to the acid, and 

 precipitated it, He then gave this process as a mean of 

 ascertaining the quantity of air contained in water. Great 

 as is the authority of this illustrious chemist, i must take the 

 liberty of observing, that, even were this phenomenon owing 

 to a superoxidation by means of the air contained in the 

 water, this mode of appreciating the quantity would not be 

 exact; because the quantity of precipitate does not depend 

 solely on the red oxide formed by the air, but rather on the 

 degree of acidity of the sulphate, which must always be sup- 

 posed uniform according to Seheele' s theory, but this is con* 

 tradicted by experience. Thus admitting the superoxiding 

 action of the air, a quart of water poured on a pound of very 

 acid green sulphate would let fall no precipitate; while § 

 quart of the same water would throw down a considerable 

 quantity from another sulphate little or not at all acid. 



The explanation of this phenomenon too is defective in 

 ijtself, for it takes place equally whether the water contain 

 air, or lie perfectly Free from if. I made this comparative 



ex peri- 



