C 34-3 3 
from fliining fubftances, and not motion propagated 
through a fluid medium ; for feveral reafons which he 
has given in his Opticks. Notwithftanding which, 
it has been urged fince his time, that light is nothing 
but a repellent fluid put into very violent vibrations. 
Now it appears impoflible, to me, at lea A, if light be 
nothing but motion propagated through a fluid medi- 
um, and not particles emitted from the luminous 
body, to account for the phenomena in the fifth, 
fixth, and feventh experiments. That a fubAance 
fliould either give light or not, when its parts are 
agitated by the fame degree of heat, according as it has, 
or has not been expofed to light, for a few feconds of 
time, more than fix months before ; feems plainly to 
indicate a Arong attraction between that fubAance 
and the particles of light ; by which it keeps many 
of them, in the common heat of the air, a long time, 
if not always : for the light the phofphorus gives by 
being heated to a certain degree appears to be caufed 
by its throwing off adventitious particles, and not by 
any of its own ; fince its light will decreafe and be en- 
tirely gone, before the phofphorus will be hot enough 
to fhine of itfelf, or to emit particles of light from its 
own body. 
A writer againA the Newtonian doctrine of light 
is prefled with a great difficulty, and afks, if it be 
poflible that a particle can move fo far as from the 
Sun to the Earth, and not frequently impinge upon 
other particles, when, he fays, every part of fpace 
mu ft contain thoufands of them ? But this difficulty 
will nearly vanifti, if a very finall portion of time be 
allowed, between the emiflion of every particle and 
the 
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