EECEEATIYE SCIENCE. 



351 



■well or cistern, from which it is ladled into 

 the oblong moulds which give it the form of 

 pig-lead. 



As the galena, no matter how carefully 

 washed, invariably contains a certain quantity 

 of the matrix — generally quartz — with which 

 it was allied in the natural state, a slag, con- 

 sisting of this substance in the melted con- 

 dition, always remains in the furnace, and is 

 run off by a distinct opening. This slag is 

 far from being worthless, seeing that it still re- 

 tains a considerable quantity of lead, in order 

 to extract which it is mixed with lime and 

 subjected to the intense heat of a blast 

 furnace. The lead thus obtained is usu- 

 ally much inferior to that obtained by the 

 first operation, and is reserved for making 

 shot — a purpose for which it is even better 

 suited than lead which is softer and less 

 flexile. 



Galena also nearly always contains traces 

 of other metals besides lead, but the only one 

 of these which it is profitable to extract is 

 silver. The proportion of this metal which 

 remains in the lead after smelting, varies in 

 different places between three and three hun- 

 dred ounces per ton ; indeed in some cases the 

 lead is totally neglected for the sake of the 

 silver. In the ores, however, to which the 

 present paper relates, the most usual propor- 

 tion is from five to twelve ounces. 



The process by which silver is separated 

 from lead is one which has, from time imme- 

 morial, been employed for obtaining the pre- 

 cious metals free from admixture with those 

 of less value ; and as we have no evidence 

 that the process has been improved upon from 

 the time of Moses up to our own, the belief in 

 a much more advanced state of chemical 

 knowledge than is generally supposed to have 

 existed at so early a period is considerably 

 strengthened. The separation of silver by 

 what is termed cupellation, is based upon 

 the circumstance, that metallic lead enters 

 into combination with oxygen at a red heat, 

 and that silver does not. Now, if an alloy 



of these two metals be melted in a vessel 

 made of some porous substance, and a strong 

 current of air be made to pass over the melted 

 mass, the lead alone will be oxidized, and 

 litharge or protoxide of lead will be formed, 

 which will be absorbed into the porous sub- 

 stance, and leave the metalhc silver unal- 

 loyed upon its surface. Now when lead con- 

 tained only a very small per centage of silver, 

 a tedious process like this, and one, more- 

 over, which required a large expenditure of 

 fuel, would be manifestly unprofitable, were 

 the litharge wasted. But it is not, for, be- 

 sides having a market value of itself, the lead 

 can be recovered from it by smelting with 

 coal, the carbon of which unites with its 

 oxygen, and liberates the metal. Even as it 

 was, the silver obtained from most ores would 

 not have paid the expense of extraction, and 

 would have been most likely altogether ne- 

 glected, were it not that the presence of a very 

 small per centage of this metal renders lead 

 hard and brittle, and unfits it for many pur- 

 poses to which it is applied. Its extraction, 

 therefore, even when there was but a little 

 present, was a necessity. 



This state of things continued up to the 

 year 1829, when the late Mr. Hugh Lee Pat- 

 tison, of Newcastle-upon-Tyne, devised the 

 ingenious process which bears his name. 

 Mr. Pattison discovered that when a melted 

 alloy of lead and silver — even though the 

 latter metal should only be present in the 

 proportion of a single ounce to the ton, was 

 allowed to cool slowly, it appeared to separate 

 into two portions, one of which took the 

 crystalline form, and fell to the bottom of the 

 melting pot; while the other remained fluid". 

 An examination of these two portions showed 

 him that the portion which solidified was 

 nearly free from silver, which all remained in 

 the fluid part. Upon this discovery he based 

 a method of desilverizing, which annually 

 adds many thousand ounces to our supply of 

 this valuable metal, and improves the quality 

 and lessens the price of lead. In illustration 



