15 
the Theory of Light and Colours. 
« much after the manner that air is compounded of the phleg- 
« matic body of air, intermixed with various vapours and 
“ exhalations : for the electric and magnetic effluvia, and gravi- 
“ tating principle, seem to argue such variety/' (Birch: Hist, of 
R. S. Vol. III. p. 249. Dec. 1675.) 
“ Is not the heat (of the warm room) conveyed through the 
<e vacuum by the vibrations of a much subtiler medium than air r 
« — And is not this medium the same with that medium by which 
“ light is refracted and reflected, and by whose vibrations light 
“ communicates heat to bodies, and is put into fits of easy re- 
« flection, and easy transmission ? And do not the vibrations of 
i( this medium in hot bodies, contribute to the intenseness and 
<e duration of their heat ? And do not hot bodies communicate 
“ their heat to contiguous cold ones, by the vibrations of this me- 
“ dium propagated from them into the cold ones ? And is not this 
“ medium exceedingly more rare and subtile than the air, and 
e( exceedingly more elastic and active ? And doth it not readily 
“ pervade all bodies ? And is it not, by its elastic force, expanded 
“ through all the heavens ? — May not planets and comets, and 
“ all gross bodies, perform their motions in this ethereal me- 
“ dium ? — And may not its resistance be so small, as to be 
t( inconsiderable? For instance, if this ether (for so I will call 
“ it) should be supposed 700,000 times more elastic than our 
tc air, and above 700,000 times more rare, its resistance would 
“ be about 600,000000 times less than that of water. And 
“ so small a resistance would scarce make any sensible altera- 
“ tion in the motions of the planets, in ten thousand years. 
“ If any one would ask how a medium can be so rare, let him 
“ tell me— how an electric body can by friction emit an exha- 
“ lation so rare and subtile, and yet so potent ? — And how the 
