accelerated and retarded by the Vibrations overtaking them.
_Qu._ 18. If in two large tall cylindrical Vessels of Glass inverted,
two little Thermometers be suspended so as not to touch the Vessels, and
the Air be drawn out of one of these Vessels, and these Vessels thus
prepared be carried out of a cold place into a warm one; the Thermometer
_in vacuo_ will grow warm as much, and almost as soon as the Thermometer
which is not _in vacuo_. And when the Vessels are carried back into the
cold place, the Thermometer _in vacuo_ will grow cold almost as soon as
the other Thermometer. Is not the Heat of the warm Room convey'd through
the _Vacuum_ by the Vibrations of a much subtiler Medium than Air, which
after the Air was drawn out remained in the _Vacuum_? 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 Reflexion and easy Transmission? And do not
the Vibrations of this Medium in hot Bodies contribute to the
intenseness and duration of their Heat? And do not hot Bodies
communicate their Heat to contiguous cold ones, by the Vibrations of
this Medium propagated from them into the cold ones? And is not this
Medium exceedingly more rare and subtile than the Air, and exceedingly
more elastick and active? And doth it not readily pervade all Bodies?
And is it not (by its elastick force) expanded through all the Heavens?
_Qu._ 19. Doth not the Refraction of Light proceed from the different
density of this thereal Medium in different places, the Light receding
always from the denser parts of the Medium? And is not the density
thereof greater in free and open Spaces void of Air and other grosser
Bodies, than within the Pores of Water, Glass, Crystal, Gems, and other
compact Bodies? For when Light passes through Glass or Crystal, and
falling very obliquely upon the farther Surface thereof is totally
reflected, the total Reflexion ought to proceed rather from the density
and vigour of the Medium without and beyond the Glass, than from the
rarity and weakness thereof.
_Qu._ 20. Doth not this thereal Medium in passing out of Water, Glass,
Crystal, and other compact and dense Bodies into empty Spaces, grow
denser and denser by degrees, and by that means refract the Rays of
Light not in a point, but by bending them gradually in curve Lines? And
doth not the gradual condensation of this Medium extend to some distance
from the Bodies, and thereby cause the Inflexions of the Rays of Light,
199