prior to the Introduction of the Potentials. 31 



numbered Niccolo Cabeo (b. 1585, d. 1650), an Italian Jesuit 

 who was. perhaps the first to observe that electrified bodies repel 

 as well as attract ; the English royalist exile, Sir Kenelm 

 Digby (b. 1603, d. 1665); and the celebrated Robert Boyle 

 (b. 1627, d. 1691). There were, however, some differences of 

 opinion as to the manner in which the effluvia acted on the small 

 bodies and set them in motion towards the excited electric; 

 Gilbert himself had supposed the emanations to have an inherent 

 tendency to reunion with the parent body ; Digby likened their 

 return to the condensation of a vapour by cooling ; and other 

 writers pictured the effluvia as forming vortices round the 

 attracted bodies in the Cartesian fashion. 



There is a well-known allusion to Gilbert's hypothesis in 

 Newton's Opticks.* 



" Let him also tell me, how an electrick body can by friction 

 emit an exhalation so rare and subtle,t and yet so potent, as by 

 its emission to cause no sensible diminution of the weight of the 

 electrick body, and to be expanded through a sphere, whose 

 diameter is above two feet, and yet to be able to agitate and 

 carry up leaf copper, or leaf gold, at a distance of above a foot 

 from the electrick body ? " 



It is, perhaps, somewhat surprising that the Newtonian 

 doctrine of gravitation should not have proved a severe blow to 

 the emanation theory of electricity ; but Gilbert's doctrine was 

 now so firmly established as to be unshaken by the overthrow 

 of the analogy by which it had been originally justified. It was, 

 however, modified in one particular about the beginning of the 

 eighteenth century. In order to account for the fact that 

 electrics are not perceptibly wasted away by excitement, the 

 earlier writers had supposed all the emanations to return 

 ultimately to the body which had emitted them ; but the 

 corpuscular theory of light accustomed philosophers to the 

 idea of emissions so subtle as to cause no perceptible loss ; and 



* Query 22. 



t " Subtlety," says Johnson, " which in its original import means exility of 

 particles, is taken in its metaphorical meaning for nicety of distinction." 



