PHYSICAL anp LITERARY. 21 
muft be propagated from their furfaces to- 
‘wards their central parts *. 
12. Hence likeways we underftand 
why opaque bodies are fooner heated by 
the fun-beams than tran{parent. ones; 
fince there are innumerable reflections 
land refractions within their fubfiances, ° 
befides what happen in common with *' 
franfparent bodies’ at their fuperficial 
parts. As each colorific particle of an o- 
paque body, by the reaction of the parti- 
cles of light, muft be fomewhat moved 
when the light is refleCted backward and 
forward between the fame particles, it is 
manifeft thatthey likeways muft be dri- 
ven backward and forward with a vibra- 
tory motion ; and the time of a vibration 
will be equal to that which light takes in 
moving through a particle, or from one 
particle of a body to another adjoining, 
This diftance in moft folid épaque bodies 
cannot be fuppofed greater than +z$55™ 
* I have found, by repeated trials, that the heat of 
water in deep lakes decreafes regularly from the furtace 
downwards, 
