666 LECTURE LIII. 



When a comnninication is made in any manner by a conducting substance 

 between the two coatings of a charged plate or vessel, the equilibrium is re- 

 stored, and the effect is called a shock. If the coatings be removed, the 

 plate will still remain charged, and it may be gradually discharged by mak- 

 ing a communication between its several parts in succession, but it cannot be 

 discharged at once, for want of a common connexion; so that the presence 

 of the coating is not absolutely essential to the charge and discharge of the 

 opposite surfaces. Such a coated substance is most usually employed in the 

 form of ajar. Jars were formerly filled with water, or with iron filings; the 

 instrument having been principally made known from the experiments of 

 Musschenbroek and others at Leyden, it was called the Leyden phial; but 

 at present a coating of tin foil is commonly applied on both sides of the jar, 

 leaving a sufficient space at its upper part, to avoid the spontaneous discharge, 

 which would often take place between the coatings, if they approached too 

 near to each other ; and a ball is fixed to the cover, which has a communication 

 with the internal coating, and by means of which the jar is charged, while 

 the external coaling is allowed to communicate with the ground. A collect- 

 ion of such jars is called a battery, and an apparatus of this kind may be made 

 so powerful, by increasing the number of jars, as to exhibit many striking 

 effiects by the motion of the electric fluid, in its passage from one to the 

 other of the surfaces. 



The conducting powers of diff"erent substances are concerned, oiot only in 

 the facility with which the motions of the electric fluid are directed into a 

 particular channel, but also in many cases of its equilibrium, and particularly 

 in the properties of charged substances, which depend on the resistance op- 

 posed by nonconductors to the ready transmission of the fluid. These pow- 

 ers may be compared, by ascertaining the greatest length of each of the sub- 

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

 in preference to a given length of air, or of any other standard of comparison. 

 The substances, which conduct electricity 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, sulfur, amber, 

 resins, silk, dry wood, oils, dry air of the usual density, and the barometrical 

 vacuum. Heat commonly increases the conducting powers of bodies; ajar of 



