206 ANNUAL OF SCIENTIFIC DISCOVERY. 



same time, covered with a tight-fitting lid ; heat is then applied, and 

 when the metal is nearly red hot a large quantity of the lead in the alloy 

 will escape, and thus the mass of alloy will become much reduced in 

 size. If care be taken that the heat be not carried to too great a 

 degree, the lead which thus escapes will be found to contain but a 

 very minute quantity of silver. The alloy thus concentrated may 

 next be treated by either of the following methods : First the alloy is 

 placed in closed retorts, or muffles, and exposed slowly to a low heat, 

 and continually stirred, by which means the metal is partly oxydised 

 and falls down in fine powder ; the heat is then increased, and when 

 all the metals (except silver) in the alloy become completely oxy- 

 dised, the whole is transferred to tanks containing dilute sulphuric 

 or muriatic acid, which dissolves the oxides, leaving the silver in the 

 metallic state. Secondly, the alloy is placed in suitable retorts, or 

 distillatory apparatus, formed of Stourbridge clay, or of iron set in 

 clay retorts and lined with powdered bone and charcoal, and by which 

 means the zinc is distilled off in the usual way, after which the back 

 part of the retort is tapped and the residue treated by cupellation, in 

 the way well known. 



By the process of desilvering lead, known as that of Pattinson's, 

 many of the English lead mines which are now workable with profit, 

 must otherwise have been abandoned. The chief ore from which lead 

 is extracted is that known as galena, or the sulphuret of lead, furnish- 

 ing from 75 to 80 parts of the metal according to purity. It usually, 

 though not always contains silver in various proportions. Upon the 

 quantity of silver often depends the profitable raising of the ore. 

 Previous to the invention of Mr. Pattinson, about 20 ounces of silver 

 in a ton of lead were required to render the extraction of that metal 

 worth the cost ; since then as little as three or four ounces in the ton 

 of lead will repay extraction. Now, as so many ores contain small 

 quantities only of silver, the importance of the process is evident. 

 In a scientific point of view it is one of much interest, as it consists in 

 so conducting the work that portions of lead can be crystallized, by 

 which the silver becomes excluded, in the manner in which in many 

 crystallizing processes, foreign substances are excluded during crystal- 

 lization, thus by degrees a mixed mass of silver and lead is left, 

 extremely rich in the first metal. When this richness in silver arrives at 

 the point desired, that metal is extracted in the usual manner by 

 cupellation. In one of the lead works in England, in which arrange- 

 ments exist by which the fumes of the furnaces are prevented from 

 escaping, the damage to the surrounding country is obviated, and lead 

 to the amount of 33 per cent, is obtained from the deposits or " fume." 



ZINC OXIDE AS A PIGMENT. 



THIS branch of industry has already arrived at much importance 

 in this country through the action of the Xew Jersey Zinc Company, 

 whose works are dependant upon the red zinc ore and Franklinite so 

 long known to mineralogists. The products of this manufacture are 



