110 On the FrarikUnian Theorij 



vallo). The appearances in an exhausted receiver, men- 

 tioned under Proposition 1. may be adduced as additional 

 proofs. Mr. Cavallo mentions a curious though delicate 

 L-xperiment as a iiirther corroboration. After moistening 

 the outside of a small phial he charged it at the prime con- 

 ductor, and while the machine is acting a beautiful brush 

 becomes visible, turning downwards to'^ards the outside 

 coating : if the outside be charged positive, the brush will 

 appear directing its course inwards. The variatioH in the 

 sensations of receiving a spark affords another collateral tes- 

 timony to these distinctions, the negative sparks being nmch 

 more pungent than the positive. The repulsion of two pith 

 balls, or gold leaf, connected with the known propertv of 

 certain substances to produce on excitation the same elec- 

 trical quality, aftords another and a more delicate and useful 

 means of detectino- minuter portions. If an electrometer 

 be brought into contact with any clecirilicd body, the balls 

 or leaf will immediately separate ; but wc wish to ascertain 

 with which electricity^ : upon exciting a piece of sealing- 

 wax and presenting it to the instrument, its divergency will 

 cither increase or diminish : if it increases, it may be con- 

 cluded similar to that of the excited substance ; if it con- 

 verges, the contrary. If yovi are desirous to obtain still 

 more accuracy of examination to determine the quality, 

 electrify two similar electrometers, one with some excited 

 body, the other with the body you wish to examine, till 

 the balls indica.te equal degrees of divergency : then bring 

 together the two electrometers : if they repel, their electri- 

 city may be deemed similar ; if they attract, opposite. 



The concurrence therefore of these two modes of exa^ 

 mination by electric light ajic! electric attraction and repul- 

 sion, if uniform and n^vari.ibb, as they are asserted to be, 

 affords a satisfactory criterion of the quality. 



We are now arrived at the eighth and last proposition, 

 which states in what the qualities of positive and negative 

 electricity consist, viz. in accvunulatioa and deficiency. It 

 has been objected, and I think with some degree of justice, 

 that this position is wholly hypothetical ; that no direct and 

 unequivocal proof can be adduced; and that the arguments 

 commonly urged in its support are included in what logi- 

 cians denominate a vicious circle. Thus it is said a body 

 positively electrified will attract another body negatively 

 electrified, because the one is redundant, the other is defi- 

 cient. But how is this known? Thev attract each other. 

 Again, in charging a phial, there is as constant a stream of 

 tlectric ftiiij fi6nj the outside coating as from the condnctor 



to 



