GOLD, SILVER, AND STEEL. 309 



Silver nitrate, or lunar caustic, as it is called when cast 

 into sticks, is, as is well known, used in photography ; 

 and from defective pictures, clippings, sweepings, and 

 washings, one firm has recovered two bars of silver, worth 



44- 



A carpet which covered the floor of one of the rooms 

 in the mint of San Francisco for five years was, when 

 taken up, cut in small pieces, and burnt in pans, with 

 the result that its ashes yielded gold and silver to the 

 value of 2,500 dollars. 



From the silver found on the copper-sheathing of a 

 ship it has been calculated that there must be about 

 200,000,000 tons dissolved in the waters of the ocean ; 

 while of gold it is thought there must be one grain in 

 every ton of water. Both these metals must, we suppose, 

 have been carried into the sea by the rivers, but no practical 

 way of recovering them has been invented. 



We have already noticed the large number of particles 

 of steel and iron in the dust of railway carriages; but 

 though less obvious, there must also be great quantities 

 in the dust of our streets, worn from wheel-tires, horse- 

 shoes, and the nails in our boots. Some day, perhaps, 

 it will be found possible and profitable to extract this 

 metal, before the mud is sent off to farmers and brick- 

 makers. 



In smelting impure ore some other mineral is melted 

 with it, to form what is called a flux. Thus iron ore 

 is melted with limestone, and the lime combining with the 

 silica of the ore, sets the iron free. The silicate of lime 

 thus produced is called slag, and great hills of it are often 



