CHAP. X. 



Of Ether, and of the Weight and Elasticity of the Air, 



I.TI 



HE moderns understand by ethar, a ?ery 

 fluidbeyond the atinosphere,and penetrating ^infinitely 

 more subtle than the air we respire, of an immense 

 extent,^ filling all the spaces where the celestial bodies 

 roll, yet making no sensible resistance to their mo- 

 tions. The existence of such a fluid is generally ac 

 knowledged, although many authors, even among 

 the moderns, differ about its nature ; some supposing 

 it to be a sort of air, much purer than that which in- 

 vests our globe ; others maintaining, that it is^ sub. 

 stance approaching to that of the celestial fire, which 

 emanates from the sun and other stars ; others make 

 it generically different from all other matter, and its 

 parts finer than those of light ; alleging, that the 

 exceeding tenuity of its parts,render it capable of that 

 vast expansive force, which is the source of all that 

 pressure and dilatation whence most of the phenomena 

 in nature arise : for, by the extreme subtilty of its 

 parts, it intimately penetrates all bodies, and exerts its 

 energy every where. 



2. But, whatever be the sentiments now enter- 

 tained with respect to the existence and nature of 

 ether, we find the origin of them all in what the an- 

 cients have said on this subject. The Stoics first of 

 all taught, there was a subtle and active fire, which, 

 diffused itself through, and pervaded the whole un'u 



