134 ELECTRICITY. 



A mineral substance, brought from the east by the Dutch navigators, called 



S by the natives of Ceylon, where chiefly it was found, Tournamal, and since 



? known as Tourmaline, exhibited, under certain circumstances, a property 



\ similar to that of amber, and other electrics. But the power was excited in it 



/ by mere elevation of temperature. Lemery, the Due de Noia, Wilson, Priestley, 



s and others, made experiments on this mineral, and published results, in which 



/ there were much discordance and contradiction. ./Epinus first showed that the 



( attraction and repulsion exerted by this gem when exposed to heat were owing 



) to the development of electricity upon it ; and that, when so excited, its op- 



S posite sides or ends had contrary kinds of electricity, one being always nega- 



? tive and the other positive. This was the first case of the distinct exhibition 



S of electrical polarity. Canton observed that the development of the electric 



/ fluid upon it was produced only by change of temperature, and that whenever 



S the gem was broken each fragment exhibited the same electrical polarity. 

 } At this period effects were observed, which, if chemical science had attained 



\ a sufficiently advanced state, could not fail to have led to the discovery of 



( electro-chemistry. Beccaria, by the electric spark, decomposed the sulphuret 



) of mercury, and recovered the metals, in some instances, from their oxides.* 



c Watson found that an electric discharge passing through fine wire rendered it 



5 incandescent, and that it was even fused and burned. Canton, repealing these 



< experiments with brass wire, found that, after the fusion by electricity, drops 



> of copper only were found, the zinc having apparently evaporated. Beccaria 



< observed that when the electric spark was transmitted through water, bubbles 

 ) of gas rose from the liquid, the nature or origin of which he was unable to de- 



< termine. Had he suspected that water was not what it was then supposed to 



> be, a simple elementary substance, the discovery of its composition could 



< scarcely have eluded his sagacity. 



) After general laws have once been developed, and their application to par- 



( ticular phenomena has become familiar, it appears wonderful that even quick- 



> sighted and acute observers should have had such effects continually repro- 

 ( duced under their eyes, without even making an approach to the discovery of 



> their causes. Franklin found that the frequent application of the electric spark 



< had eaten away iron ; on which Priestley' observed, that it must be the effect 

 ) of some acid, and suggested the inquiry, whether electricity might not probably 



< redden vegetable blues ? Priestley also observed that in transmitting electricity 

 ) through a copper chain, a black dust was left on the paper which supported 



< the chain at the points where the links touched it ; and, on examining this 

 ) dust, he found it to contain copper. 



< Some years after the invention of the Leyden jar, when the necessity of 



> some sufficient indicator of the presence of electricity, and some visible meas- 

 ( ure of its power became apparent, the invention of electrometers engaged the 

 / attention of electricians. After several abortive attempts on the part of others, 

 ^ the Abbe Nollet proposed the simple expedient of suspending two threads, 

 ? which, when electrified, would separate by their mutual repulsion. Cavallo 

 S afterward improved upon this, by substituting two pith balls, suspended in con- 

 j tact by fine metallic wires an apparatus still used. After this, various formr 

 j of electroscopic instruments were suggested and constructed by Volta, Saus- 

 ? sure, and others, all depending on the principle that the intensity of the elec 



S trie fluid was manifested by the force of its attraction or repulsion exerted upon 



? light substances to which it was imparted. 



j The principle of induction applied to the air-condenser by Wilke and ^Epi- 



? nus, was taken up by Volta, and applied, first, to the constructor of the ELEC- 



* Leltere del Elettricismo, $ 341, p. 282. 



