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axial centres, so as to bring a new point of the circumference into 

 play, should abrasion or any other defect arise in the existing ex- 

 ploding point. The author endeavours to show that the apparent 

 irregularities so frequently observed in the striking distance of a 

 charged electrical jar, do not arise from any defect in the quantity 

 measure, or in the exploding electrometer when properly constructed, 

 but are altogether dependent on some peculiar conditions of electrical 

 accumulation on coated glass. 



One remarkable peculiarity of the electrical jar, is a disposition to 

 retain a portion of the charge notwithstanding explosive discharge 

 has occurred through a discharging circuit ; we do not discharge the 

 whole accumulation ; a portion is, as it were, left behind. The fact 

 itself is undisputed, but the cause or theoretical explanation does not 

 appear to have been very clearly comprehended. 



The author here introduces some interesting quotations from 

 certain unpublished manuscripts of Mr. Cavendish, who investigated 

 so long since a* the years 1771 and 1772, what he terms the 

 "charges of plate glass and other electrical substances coated in 

 the manner of Leyden phials." Mr. Cavendish found his experi- 

 mental inquiries greatly embarrassed by the " spreading of the elec- 

 tricity" on the glass; it is, he says, faster on some kinds of glass 

 than on others ; besides the slow and gradual spreading, he observed 

 an instantaneous spreading, visible in the dark, and extending to 

 about '07 of an inch beyond the edge of the coating upon glass *2 

 of an inch thick, and about '09 upon glass ygth of an inch thick. 

 Another source of inconvenience, observes Mr. Cavendish, arises from 

 a certain amount of penetration of the charge into the substance of 

 the glass itself, equal to about -j^-ths of its thickness ; the space, he 

 says, within which the charge cannot penetrate is not above -i-th of 

 the thickness, from which he concludes that the charge of a coated 

 electric will be different in cases in which this penetration of the 

 charge into the substance of the glass varies, and infers that different 

 electrics are susceptible of different degrees of charge. He examined 

 plates of glass of various kinds, as also of gum-lac, rosin, bees-wax, 

 &c., and found the capacity of these substances for electrical charge 

 different phenomena recently explained by Faraday's fine discovery 

 of Specific Inductive Capacity. 



This last celebrated philosopher also recognizes the penetration or 



