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taining a large amount of the oxides of iron, and also 
organic matter, which, by their mutual re-action, gave rise to 
a solution of bicarbonate of iron — that this solution per- 
colated through the limestone, and, removing a large part 
of the carbonate of lime by solution, left in its place 
carbonate of iron ; and not that the rock was formed as a 
simple deposit at the bottom of the sea. 
ON THE PURIFICATION OF LEAD BY CRYSTALLIZATION. BY 
WILLIAM BAKER, ESQ., OF SHEFFIELD. 
The following Paper is to shew chiefly that copper may be 
separated from lead by the process of crystallization intro- 
duced in 1833, by Hugh Lee Pattinson, for the purpose of 
desilverizing lead, which process is now very generally em- 
ployed in England and on the continent. 
Pattinson was led to this discovery by the accidental 
upsetting of a pot containing argentiferous lead, which had 
cooled down nearly to the point of solidification. Observing 
a portion more fluid than the rest ; he found upon assaying 
both, that it contained the larger proportion of silver. 
The principles of the process are these : — When lead con- 
taining a certain amount of silver is melted and then allowed to 
cool slowly, whilst it is constantly stirred, a crystallization of 
pure lead obtains at a temperature at which an alloy of silver 
and lead still remains fluid. The crystals of lead which sink 
towards the bottom of the vessel may be removed by means 
of a perforated ladle, leaving the rich lead, (that is lead con- 
taining an alloy of silver and lead,) to be further treated in a 
similar manner until the silver is concentrated sufficiently to 
be recovered by the old method of cupellation. 
Formerly, lead containing less than 20 oz. of silver per 
ton would hardly bear the cost of extraction. By Pattinson's 
process, with a sufficiently extensive battery of pots, lead 
containing only 2\ to 3 oz. per ton, may be worked with 
