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teaches, that the iron, diffolved in water by fixed air, 
is at lead; equal in quantity to what is commonly 
afcribed to mofl chalybeate fprings : that this air, by 
which the metal is held in folution, is fimilar to that 
elaftic vapour, fo often mentioned by writers on thefe 
fubjeds ; which cannot be wholly retained by the 
clofeft corking, but, gradually efcaping, fuffers the 
ochrous matter to fubfide. And that fixed air has 
greater affinity with alcalies than with iron, becaufe 
addition of alcaline fubflances, not faturated with 
fixed air, will difengage the metal, while fuch as are 
charged with this principle produce no alteration^ 
Thefe cojiclufions feemed to account for many 
particulars relating to medicated fprings ; but as all 
my trials had been made with iron in its metallic 
flate, which is rarely found in nature, it was necefi* 
fary to repeat them upon this mineral in the flate of 
ore. I proceeded, therefore, to different experiments, 
upon various ores j but did not find any of them to 
anfwer my expectation, except what is called fion 
fand ore, which feems to contain a perfeCl iron. 
This, at firft, offered a material objection to my 
former inference. But, upon a little confideration, it 
occurred, that waters, being firft charged with pyri- 
tical matter, might afterwards have their acid neu- 
tralized with alcaline or calcarious fubflances, and the 
iron yet remain fufpended by air generated in the fa- 
turation. And I was the more ready to adopt this; 
opinion, as it would explain, very naturally, the ap- 
plication of fixed air to this bufinefs of folution j. 
which, I confefs, had hitherto been to me forae- 
what difficult to account for. It was neceffary, 
how'ever, to examine the truth of this theory, by 
