448 Dr. Percival on Attra£lion and Repu'fioH, 



of fubfiding. Refin, butter, or tallow, whert 

 liquefied by the heat of the boiling brine, float 

 upon its furface, and will rennain perfe6tly fmooth 

 and undifturbed, whilft the v/ater, beneath, may 

 be put into ftrong agitation, by the adtion of the 

 . fire. Such agitation muft break down the cry- 

 ftals of fait, as they fhoot ; and confequently^ 

 only fmall granules will be produced. 



II. Every one has experienced the fuffbcating 

 effects of air, loaded with the effluvia of burnt 

 greafe, or the fnuff of a lamp. When fuch 

 fumes are infpired, there is the lenfation of 

 a conflict in the lungs, which effentially differs 

 from what is felt, on breathing either fixed or 

 inflammable air. And is not the mod eafy 

 folution of it, to fuppofe, that the air quits the 

 oily, to unite with the watery vapours, which 

 ^re brought into contiguity, by this adion of 

 the animal ceconomy ; and that a ftrong re- 

 puifion fucceeds ? " For, as, in algebra, where 

 ** affirmative quantities vanilh and ceafe, where 

 " negative ones begin, fo, in mechanics, where 

 ** attradion ceafes, there a repulfive power ought 

 *' to fucceed," according to the dodtrine of Sir 

 Ifaac Newton. It is, alfo, an axiom, laid down 

 by this great philofopher, that *' to the fame 

 " natural effects, we mufl: always affign, as 

 " far as poffible, the fame caufes." I fhall 

 therefore proceed to illuftrate this fubjc6l, by 

 other more decifive examples of repul/icn; after 



premifing 



