Ohfervations on the Theory ofEleSric AltraElion. 463 



negatively ele(3:rified. The former was attracted towards the latter until it arrived at a 

 certain diftance, at which it difcharged its eledricity. Hence he concluded, in general, 

 that when a body has more of the cleftric fluid than the neighbouring bodies, and is lefs 

 difpofed to refill its own motion than to abandon the excefs of its eleftric matter, it will 

 move towards that place which contains lefs of this matter. But in this experiment he 

 conGders the two bodies as zOivag on each other at a diftance without any reference 

 to the intermediate air. 



Mr. Cavallo *, in the laft edition of his treatife on eleftricity, has obferved, that the 

 mutual repulfion of two bodies negatively ele£l:rif;ed is ftill fuppofed to contradidi the 

 theory of Franklin ; and has therefore deemed it neceflary to obviate the objedlion by a 

 very particular detail. For this purpofe he has premifed the following propofitions : 

 Prop. I. No ele£lricity can appear on the furface of a body, or no body can be eledlrified 

 either pofitively or negatively, unlefs the contrary ele£lricity can take place on other bodies 

 contiguous to it. Prop. 2. There is fomething on the furface of bodies, which prevents 

 the fudden incorporation of the two eleftricities, viz. of that poflefled by the ele£trified 

 body with the contrary eleftricity poflelTed by the contiguous air, or other furrounding 

 bodies. Prop. 3. Suppofing that every particle of a fluid has an attraction towards every 

 particle of a folid ; if the folid be left at liberty in a certain quantity of that fluid, it will 

 be attradled towards the common centre of attraction of all the particles of the fluid. To 

 this laft propofition he has fubjoined the two following corollaries: i. f the fame thing 

 muft happen, when the quantity of fluid is fmaller than the bulk of the body ; 2. if the 

 attradlion of the particles of the fluid be exerted only towards the furface of the folid, the 

 eftc£t will be the fame when the body is of a regular Ihape ; but the difference will 

 in any cafe be inconfiderable. 



With regard to the folution founded upon thefe principles it muft be remarked, that it 

 is not derived fimply from a confideration of the fuppofed nature of the ele£tric fluid; but 

 from a mixed ftatement of that nature and of properties aflumed merely from experiments 

 as matters of faft. The firft and fecond propofitions exprefs thofe properties, and, though 

 the experiments to which the former refers, may be explained by afcribing the phsenomena 

 to the repulfive nature of the fluid, yet the latter is aflTumed without any fuch reference. 

 •' Without examining," fays Mr. Cavallo, " the nature, the extent, and the laws of this 

 " property in bodies, it will be fufiicient for the prefent purpofe to obferve, that the fafl: 

 ** is certainly fo ; for otherwife a body could not pofiibly be eledtrified, or it would not 

 " remain electrified for a fingle moment." From thefe principles thus afl^umed, Mr. 

 Cavallo deduces the exiftence of atmofpheres of contrary eledlricity exifting in the air 

 contiguous to the bodies ; and from the attra£l:ions which are thereby occafioned he infers 

 the apparent repulfion of the eledlrified bodies. 



• Vol. III. p. 191. 



t Of this corollary Mr. Cavallo does not appear to make any dillinft application. 



If 



