4:3 Benefits from tie Gravity of Air. [Bock V. 



ther form, that is open at both ends, and covered with 

 a bladder at the top, and beginning to exhauft the air, 

 when the bladder will infallibly be burrt by the pief- 

 fure of the exteiior atmofpheric air. 



This gravity of the atmofphere accomplices many 

 vft'ful purpofes in nature, fuch as preventing the blood 

 veffels of animals, and the fap veffels of plants, from 

 being too much diflended by the expanfive power, 

 which has a perpetual tendency to fwell them out. 

 On this account we fee, that in the operation of cup- 

 ping, where the preffure of the air is taken off from 

 a particular part, the expanfive force inftantly acts, 

 and fwells out the veiTcls to a great degree. This is 

 alfo the rcafon why the bodies of animals fwell when 

 they are put into an air-pump. It is owing to the 

 gravity ,of air that fubftances remain liquid, which 

 would become aeriform in vacuo. Salts and oils re- 

 main united in air, but feparate as foon as that fluid 

 is extracted. When hot water is put under an ex- 

 haufted receiver, it boils violently j becaufe the pref- 

 fure of the air being now taken off, there is nothing 

 to prevent it from affuming the (late of vapour. 



4 This preflure of the armofphere,' fays Lavoifier, 

 f caufes- water to remain in a liquid (late till it is 

 raifed to 212 of Fahrenheit's thermometer, the quan- 

 tity of heat which it receives in a lower temperature 

 being inefficient to overcome the preffure of the at- 

 mofphere. Whence it appears, that without this pref- 

 fure we fhould not have any permanent liquid, ancj 

 fiiouki only be able to fee bodies in that (late of exif- 

 tence in the very inflant of melting, as the (mailed 

 additional heat would inftantly feparate their particles, 

 and diflipate them through the furrounding medium. 

 Befides, without this atmofpheric preffure, we mould 

 not even have any aeriform fluids, (Iridly (peaking, 



becaufe 



