MAGNETISM. 



417 



345, may be mado to develop electricity by heating Fio. 345. 



one end of the bundle, or pile of plates. 



By binding togL-ther two bars of bismuth and 

 antimony, an electric current can be proved to circu- 

 late with the slightest variation of temperature. 



A series of slender bars of these two metals, ar- 

 ranged as a thermo-electric battery, is far more sen- 

 sitive to heat than the most delicate thermometer; 

 so that the heat radiated from the hand brought near 



to one end of the battery is sufEcient to excite an appreciable amount of eleo- 

 tricity. 



Fig. 346 represents the construction of such a battery. It consists of thirty- 

 six delicate bars of bismuth and antimony, 

 alternately connected at their extremities 

 and packed in a case, the ends of which 

 are removed ia the figure to show tho 

 bars. The area of such a battery is not 

 quite one half an inch. A represents a 

 conical reflector, used to concentrate rays 

 of heat in experimenting. 

 It has been also found that when hot water mixes with cold water, that 

 electricity is produced ; tho hot hquor being positive and tho cold negative. 



Fig. 346. 



CHAPTER XVIII. 



MAGNETISM. 



What iB a nat- ^^'^- -^ NATURAL magnet, sometimes called 

 nrai magnet? j^ loadstone, IS Ell 016 of iroii, knowu as tho 

 protoxyd of iron, or magnetic oxyd of iron, which is ca- 

 pable of attracting other pieces of iron to itself. 



_. _ Natural magnets are by no means rare ; they 



are found in many places in the United States, 



and in Arkansas, especially, an ore of iron pos- 



^^^^^^^^^ 1 sessing remarkably strong attractive powers is 



A^^^^^^^^^^H^^l/ very abundant. 



jR^^^^^^^^^^^HHr" The magnetic ore is usually of a dark color, 



Hi Y^^^Jj^^^^^^V y^ and possesses but little metallic luster. If a 



'l\\ I "^ piece of this ore be dipped in iron filings, or 



brought in contact with a number of small 



18* 



