51 



ELECTRICITY. 



three -sided pyramid at one end, and by 

 a six-sided pyramid at the other. Le- 

 mery noticed its electric properties in 

 the year 1717; but the first scientific 

 examination of them was made by 

 .^Epinus in 1756, and published in the 

 Memoirs of the Berlin Academy. He 

 found that when a crystal of tourmalin 

 has its temperature raised to between 

 100 and 212 of Fahrenheit, one ex- 

 tremity, which is that terminated by the 

 six-sided pyramid, becomes charged 

 with positive electricity, while the other 

 extremity is negative ; so as to be capa- 

 ble of affecting a delicate electroscope. 

 When the stone is of considerable size, 

 flashes of light may be seen along its 

 surface. Mr. Wilson, who made many 

 experiments on this subject, observed 

 that a flat tourmalin retained its electri- 

 city without diminution, after exposure 

 to intense heat for half an hour ; but 

 Canton, upon repeating these experi- 

 .ments, did not obtain the same result. 

 Hauy states, that very high degrees 

 of heat destroy the electricity of the 

 tourmalin. After this has been effected, 

 it recovers its electricity as it gradually 

 cools : but in that case the electric states 

 are generally reversed ; that extremity, 

 or pole, as it has been called, which 

 was before positive, is now negative, 

 and vice versa. It is only at the sum- 

 mits of the pyramids, by which the 

 crystal is terminated, that the electricity 

 is manifested ; the intermediate por- 

 tions exhibiting no sign of electrical ex- 

 citation, unless the stone be broken in 

 pieces ; and then each fragment is 

 found to possess a positive and a nega- 

 tive pole, like the entire crystal. This 

 fact bears a striking analogy to a cor- 

 responding property in magnets. At 

 the ordinary temperature of the atmo- 

 sphere, the tourmalin may be rendered 

 electrical by friction. 



(197.) There are several other gems 

 and crystallized minerals which possess 

 the same property as the tourmalin. 

 The luminous appearance of some dia- 

 monds, when heated, is ascribed by 

 Sir Humphry Davy to their electrical 

 excitation. The substance called the 

 Boracite, composed of borate of mag- 

 nesia, which crystallizes in cubes, having 

 its edges and angles defective, becomes 

 electrical by heat, and in one variety 

 presents no less than eight sides, alter- 

 nately in different states ; that is, four 

 positive and four negative ; the oppo- 

 site poles being in the direction of the 

 axes of the crystal. In those varieties 

 in which only four of the angles of the 



crystal are truncated, that is, cut off by 

 planes, while the rest are either entire, 

 or are replaced by more than one plane, 

 it is always the former of these angles 

 that become positive, and the latter 

 negative. 



(198.) Similar properties are possess- 

 ed by the Topaz, which consists of 

 siliceous fluate of alumina ; its electric 

 poles are situated upon the two opposite 

 summits of the secondary crystal. In 

 some varieties, Hauy found a series of 

 consecutive poles alternately positive 

 and negative. Axinite, Mesotype, and 

 Prehnite, become electrical by the ap- 

 plication of heat : as also the two fol- 

 lowing metallic oxides, namely Gala- 

 mine, which is an oxide of zinc, and 

 Sphene, or calcareo- siliceous oxide of 

 titanium. Mr. Dessaignes has lately 

 shown that all metallic bodies are capa- 

 ble of a feeble electric excitation by 

 changes of temperature. It results 

 from the researches of Hauy, that this 

 electrical property in mineral bodies is 

 intimately related to the laws of their 

 crystallization, and also to the direction 

 in which the light is most readily trans- 

 mitted through them. 



(199.) There are a great many sub- 

 stances which become electrified on 

 passing from the liquid to the solid 

 form. This happens to sulphur, gum 

 lac, bee's wax, and in general all resi- 

 nous bodies. Unless proper precautions 

 be taken, however, we frequently obtain 

 no indications of this electricity, because 

 it is usually disguised, that is, rendered 

 inactive by the opposite electricity of 

 the contiguous substances. Thus, if 

 sulphur be melted over the fire in an 

 iron ladle, and then set by to cool and 

 harden, it exhibits no sign of electricity ; 

 because the negative electricity of the 

 sulphur is exactly counterbalanced by 

 the positive electricity accumulated in 

 the iron vessel which contains it. But 

 if the sulphur be removed from the 

 vessel, which may be done by again 

 heating it for a short time, so as just 

 to melt the surface in contact with the 

 iron, and allow of its being detached 

 when the ladle is inverted, on suffering 

 the sulphur to cool in this situation, its 

 electricity becomes very apparent. If 

 sulphur be melted in a wine glass, the 

 conical shape of which admits of its 

 being taken out when cold, the opposite 

 electricities of the two surfaces will then 

 manifest themselves, that of the sulphur 

 being negative, and that of the glass 

 positive ; but when the sulphur is re- 

 place4 iu the glass, all indications of 



