345 Ele£lncify propagated in Vacuo. 



I ft, That the common air in which we live is not the 

 proper matter which condufts found, fince, notwithftanding 

 the great tranfparencv of this fluid, it is ftill too grofs to pe- 

 netrate freely the malTes of thofe bodies which have greater 

 denfity than itfelf ; a property which the matter fit for con- 

 thifting found evidently pofTefles. 



2d, That there exifls an invifible, exceedingly fubtile and 

 :fingalarly elaftic fluid, of extreme rarity, which eafily pene- 

 trates all bodies, which is diffiifed throughout every part of 

 our globe, and confequently throughout our atmofphere. 



3d, That this fluid is the eflential caufe of the elafticity 

 •with which atmofpheric air feems to be endowed, and that 

 it is to the vibrations communicated to this fubtile fluid — 

 vibrations which are tranfmitted with great celerity through 

 difierent mediums, and even the moft folid — that we ought 

 to afcribe the immediate caufe of found and noife in regard 

 to us. 



4th, That the fubtile fluid which conftitutes the propa- 

 gating matter of found is exaftly the fame as the ethereal 

 fire, of which the author fays he has demonftrated the exifl:- 

 ence in his different writings, and which may be confidered 

 as the ethereal fluid mentioned by Newton, if to its well 

 Itnown properties we do not add the fuppofition by which 

 Newton gives to its vibrations a velocity greater than that of 

 light. 



Tremery has confirmed the opinion of thofe who think 

 that electricity is propagated in vacuo. He exhaufted en- 

 tirely of air the tube of a barometer ; and having extra6led a 

 fpark by means of a metallic rod, the eleftric fluid paffed 

 in the vacuum, and the whole infide of the tube became 

 luminous. 



ATMOSPHERIC AIR, 



Humboldt has publiflied the refult of his obfervations on 

 the nature of atmofpheric air, by which it appears that the 

 purity of this air is fubjeft to great variations. The fum of 

 his obfervations is as follows : — ^The quantity of oxygen con- 

 tained in atmofpheric air decreafes according to the abun- 

 dance of clouds, fogSj rain, and fnow; but it increafes during 



dry 



