416 ] [BookV. 



CHAP. IX. 



OF THE WEIGHT, ELASTICITY, AND OTHER 

 GENERAL PROPERTIES OF THE AIR. 



General Properties of Air. Caitfe of its Elajlicity, (Opinions of the 

 Antients, Torricellian Experiment. Barometer. The Air Pump. 

 Weight and fpedjic Gravity of Air. Immenfe Preffure of the 

 Attmfphere.ComprfJpbility of Air. Cupping GlaJJei. Ejj'eds of 

 the Air's Elajlicity. -^- Air Guns, fcffc.- Motion cf Particles in o~ 

 dies.- Nature of the Atmofphere. Its probable Height* 



THE air, confidered as a fluid, without any re* 

 fpect to its component principles, has alfo fome 

 properties which are of the utmoft importance in the 

 lyftem of nature ; and the confideration of thefe pro- 

 perties will ferve to illuftrate and explain the nature of 

 all other elaftic fluids. 



Atmofpheric air, confidered in itfelf, is a ponderous, 

 compreffible, elaftic, tranfparent body, without colour> 

 invifible, and incondenfable by any degree of cold that 

 can be produced in the temperature of this earth. It 

 never becomes the conftituent part of any body; 

 though it bafes, that is, oxygen and azote, enter into 

 the compofition of many. 



The FLUIDITY of the air is caufed by the matter of 

 fire or heat, which produces in it a degree of elafticity 

 that always tends to dilate the mafs, and preferves 

 the motion of its parts. If the air was not elaftic, 

 it might be formed into a hard body, like fnow, when 

 its particles are prefled forcibly together. 



It is eafy to prove that air adheres, with a confider- 

 able degree of force, to the furface of bodies ; for when 



water 



