222 ANNUAL OF SCIENTIFIC DISCOVERY. 



lime. He also lays particular stress on the occasional presence of < 

 silica as an impurity in this mineral, interesting not only in its 

 bearing on the paragenesis of minerals, but for the explanation it 

 suggests of many difficulties hitherto experienced in reconciling 

 the results of analyses, especially of silicates. 



LEAD, SILVER, AND GOLD. 



In a lecture on Chemical Geology, delivered by Dr. Percy at 

 the Royal School of Mines, London, this singular fact is an- 

 nounced, namely: "It may be laid down as a universal proposi- 

 tion, without any exception so far as is known, that all galena 

 contains silver all. There are varieties which contain a very 

 small quantity of silver, and which are therefore said to be ' poor;' 

 still, if search be made for the metal even in the poorest kinds, 

 you never fail to obtain not only traces, but far more than traces, 

 of silver. The silver in the sulphide of lead must, of course, ex- 

 ist as sulphide of silver. Galena is a source of a very large sup- 

 ply of silver in different parts of the world. It may be laid down, 

 also, not merely as ,a general rule, but, I believe, as a universal 

 proposition, that all galena contains gold all galena. Some 

 years ago Mr. Smith and myself set to work to examine this 

 point, and we made a great many determinations with respect to 

 the presence of gold in the ore of lead, and in various commercial 

 compounds of lead. Forty specimens or more were examined, 

 and every one yielded palpable, visible, unmistakable traces of 

 gold. Still, the quantity of gold is so small as to be utterly worth- 

 less in a commercial point of view. Here are the evidences of 

 these facts. Every single specimen of gold extracted in these 

 experiments has been carefully preserved in hermetically sealed 

 tubes, and the condition specified. It requires rather careful 

 manipulation. In these experiments there was no possibility of 

 error. There was nothing added in the way of chemical reagent, 

 which might vitiate the result. The process consists of taking the 

 compound of lead, and simply submitting it to the well-known 

 operation called cupellation, by which silver is extracted from lead. 

 There remains behind a very small globule of silver, and in that 

 globule we are enabled to detect the gold by the simple action of 

 a solvent of silver, nitric acid, which leaves the gold. It has to 

 be taken up with great care, and transferred to a piece of blot- 

 ting-paper. It is afterward gummed on to a piece of paper and 

 then burnished, when the characteristic color of gold immediately 

 appears. It is a remarkable circumstance that gold is detected 

 not only in the ore of lead, but also in the various commercial 

 compounds of lead, white lead, red lead, sugar of lead; nay, 

 we have even gone farther, and found it in lead fume, that is, the 

 smoke that is volatilized from lead in the process of its extraction. 

 We may then, I think, safely conclude that lead contains always 

 silver and gold. Perhaps you may object to the deduction with 

 regard to gold as not being sufficiently supported. It is founded 

 on 40 examinations under various conditions. At all events, tiie 

 proof is strong, if not conclusive." 



