Notices respecting New Books. 435 



\vilh the ground, gains the same electricity, and the middle 

 point is nut electrical at all. This is easily proved, hy exa- 

 mining the electricity of three sets of gilt pith balls raised on 

 wires on the different pans of the conductor, which is thus 

 affected by induced electricity. If, instead of air, a plate of 

 mica or glass be between the two conductors, the same phse- 

 nomena will occur; so that it would appear that the con- 

 ductor merely gains two opposite electricities, or polar elec- 

 tricities, of the same kind as those of the nonconductor. 

 The phaenomena of sparks, of discharges, and of accumu- 

 lated electricity, depend upon this law. In the case of the. 

 common electrical spark, a stratum of air is charged in the 

 same manner as a glass bottle, partially coated with tin foil, 

 is charged in the Leyden experiment ; when the hand i.s 

 held near the positive conductor of an electrical machine, 

 the person standing on the ground, the hand is rendered 

 negative, and the states become exalted, till the polarities, 

 as they may be called, are annihilated through the air, pro- 

 ducing a spark, a snap, and a distinct sensation. If a 

 number of small pith balls, placed upon a surface of metal, 

 are caused to approach an electrified body, they are brought 

 into the opposite state by induction, and are attracted to- 

 wards the body ; but when they come in contact with it, 

 this state is destroyed, they gain the same state, and are 

 repelled." 



So far the phaenomenon of electrical polarity is perfectly 

 analogous to that of magnetism. The coincidence favours 

 the reveries of speculators relating to an electrical and 

 magnetic atmosphere. As to the term induction^ which is 

 more familiar to metaphysical than physical language, it 

 seems as convenient and applicable as any other. Mr. G. 

 Singer, indeed, in his lectures on electricity made some 

 objections to it ; but as he proposed to state his reasons in 

 the Philosophical Magazine, we shall leave his opinions to 

 his own enunciation. " In consequence of the principle 

 of induction," says Sir H. " the condensing electrometer is 

 much more sensible than the commrtn electrometer. This 

 instrument consists of two plates of polished metal, the 

 surfaces of which are parallel, one connected with the 

 pilatt of the electrometer, the other moveable, in connection 

 with the ground, and the plates are very near each other. 

 When the body supposed to be electrical, is made to touch 

 (he lop of the electrometer, and is afterwards removed in 

 separating the polished plates, the effect will be perceived." 



*' 'I'lic difference in what are called the conducting 

 powers of bodies, seems to depend entirely upon the dif- 



E e 2 ferent 



