no ESSAYS ann OBSERVATIONS 
light produces heat by adually ftriking 
the folid parts of bodies, after we are {atis- 
fied that bodies produce the refleGion and 
refraction of light, without fuffering it to 
come into contact with them 
10. From thefe principles it follows, 
that light, in pailing out of one medium 
into another of different denfity, muft al- 
ways produce fome degree of heat; be- 
caufe it is partly refracted and reflected at 
the common furface: Secondly, that, in 
pating forwards through the fame ho- 
mogeneous or perfectly tranfparent me- 
dium, it can produce no heat ; becaufe there 
is no reflection or refraction, no influence 
of the body upon the light, but every ray 
purfues its own right-lined courfe, as if it 
moved in a perfect void*. 
11, HENCE it appears, that, in water, 
glafs, and other tranfparent mediums, which 
are warmed by the fun’s rays, the heat 
muit 
¥ Sir Ifaac Newton, in the third book of his Principia, 
where he dijputes concerning the tails of comets, lays it 
Cown as an obvious principle, Quod radi: folis non agi- 
tant media quae permanant, nifi in refiexione et refrace 
vione. : 
