CHAP. I.] ELECTRICITY AND MAGNETISM. 67 



excitations and muscular contractions of human beings 

 also give rise to feeble discharges of electricity. 



Fig. 38. 



69. Electricity of Vegetables. Buff thought he 

 detected electrification produced by plant life ; the roots 

 and juicy parts being negatively, and the leaves posi- 

 tively, electrified. The subject has, however, been little 

 investigated. 



70. Thermo-electricity. Heat applied at the 

 junction of two dissimilar metals produces a flow of 

 electricity across the junction. This subject is discussed 

 in Lesson XXXIV. on Thermo-electric Currents. 



71. Contact of dissimilar Metals. Volta showed 

 that the contact of two dissimilar metals produced 

 opposite kinds of electricity on the two surfaces, one 

 becoming positively, and the other negatively, electrified. 

 This he proved in several ways, one of the most con- 

 clusive proofs being that afforded by his condensing 

 electroscope. This consisted of a gold-leaf elec- 

 troscope combined with a small condenser. A metallic 

 plate formed the top of the electroscope, and on this 

 was placed a second metallic plate furnished with a 

 handle, and insulated from the lower one by being well 

 varnished at the surface (Fig. 68). As the capacity of 

 such a condenser is considerable, a very feeble source 

 may supply a quantity of electricity to the condenser with- 

 out materially raising its potential, or causing the gold 

 leaves to diverge. But if the upper plate be lifted, the 

 capacity of the ijQjvar..|>late diminishes enormously, and 



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