ELECTRICITY AND MAGNETISM. 685 



larger in size and a very prosperous mining town. It lies in a gulch at the foot 

 of a chain of mountains, called San Francisco, and its population derived and 

 still derives its living chiefly from mining and smelting tin ores found in the 

 above mountains. The San Francisco chain is one of the lateral branches of the 

 main chain of Rocky Mountains, and is about loo miles in length and from forty 

 to sixty miles in width. Throughout its length and width it is a series of abrupt 

 peaks, rocky canons and some occasional grassy slopes and long but narrow val- 

 leys. The whole of it is heavily wooded with oak and pine, but is almost water- 

 less, although during the rainy season many unfordable mountain torrents are 

 met with there, and the grounds offer at many points facilities for damming, 

 where a short dam, if built, would entrap an extensive reservoir or a series of 

 reservoirs of water. Toward the southwestern end of this ridge is the central 

 point of the Mexican tin region. Its extent is about 150 miles in length and 

 about the same in width. 



The tin ore found there is principally red and black cassiterite of extreme 

 purity, and this is found either in veins which fissure in all directions these peaks 

 and their slopes, or in placers, in gulches and valleys at the foot of the peaks 

 and slopes. As the foot of all ridges and slopes where a vein exists, a placer of 

 tin is invariably found. Sulphuret of tin, similar to Cornwall, England, tin stone, 

 has also been found, but, so far, only in one point of these mountains ; while 

 arseniate of tin and also very finely divided red ophite of tin are met with in 

 many of the numerous veins already tapped. The massive cassiterite is usually 

 found in the veins in continuous streaks of various widths, the ore being, of 

 course, easily separated from its gangue, and containing from 70 to 80 per cent of 

 metallic tin. The veins are very numerous, and although many of them are cut 

 into, and some have been worked quite extensively, it is a certainty that only a 

 small percentage of them are known at present, and a great many more could be 

 found by judicious prospecting. 



PHYSICS. 



ELECTRICITY AND MAGNETISM.^ 



PROF. FRANCIS E. NIPHER. 



It was known six hundred years before Christ that when amber is rubbed it 

 acquires the power of attracting light bodies. The Greek name for amber, elekt^vn, 

 was afterwards applied to the phenomenon. It was also known to the ancients 

 that a certain kind of iron ore, first found at Magnesia, in Asia Minor, had the 

 property of attracting iron. This phenomenon was called magnetism. This is 



1 Introductory to the course of Lectures on Physics at Washington Uuiversity, St. Louis 

 Missouri. 



VIII— it 



