26 ELECTRICITY IN MOTION. 



compared, by ascertaining tho greatest length of each of the sub. 

 stances to be examined, through which a spark or a shock will 

 take its course, in preference to a given length of air, or of any 

 other standard of comparison. The substances which conduct 

 ricity the most readily, are metals, well burnt charcoal, 

 animal bodies, acids, saline liquors, water, and very rare air. 

 The principal nonconductors are glass, ice, gems, dry salts, sul. 

 phur, amber, resins, silk, dry wood, oils, dry air of the usual 

 density, and the barometrical vacuum. Heat commonly increases 

 the conducting powers of bodies ; a jar of glass may he discharged 

 by a moderate heat, and liquid resins are capable of transmitting 

 shocks, although they are by no means good conductors : it is 

 remarkable also that a jar may be discharged by minute agitation, 

 when it is caused to ring by the friction of the finger. It has been 

 observed that, in a great variety of cases, those substances which 

 are the best conductors of heat, afford also the readiest passage to 

 electricity ; thus, copper conducts heat more rapidly, and electri. 

 city more readily, than iron ; and platina less than almost any 

 other metal : glass also presents a considerable resistance to the 

 transmission of both these influences. The analogy is, however, 

 in many respects imperfect, and it affords us but little light, with 

 regard either to the nature of heat, or to that of the electric fluid. 



[Young's Nat. Phil. 



SECTION III. 



Electricity in Motion. 



1 :it manner in which the electric fluid is transferred from one 

 body to another, the immediate effects of such a transfer, the 

 causes which originally disturb the equilibrium of electricity, and 

 the practical methods by which all these circumstances are regu. 

 lated and measured, require to be considered as belonging to the 

 subject of electricity in motion. Among the modes of excitation 

 by which the equilibrium is originally disturbed, one of the most 

 interesting is the galvanic apparatus, which has been of late years 

 a very favourite subject of popular curiosity, and of which the 

 theory and operation will be briefly examined, although the subject 

 appears rather to belong to the chemical than to the mechanical 

 doctrine of electricity. 



The progressive motion of the electric fluid through conducting 



