^0 ESSAYS AND OBSERVATIONS 



are warmed by the fun's rays, the heat muft 

 be propagated from their furfaces towards 

 their central parts *. 



12. Hence likeways we underftand why 

 opaque bodies are fooner heated by the fun- 

 beams than tranfparent ones ; fince, there are 

 innumerable reflexions and refradions within 

 their fubftances, befides what happen in 

 common with tranfparent bodies at their fu- 

 periicial parts. As each colorific particle of 

 an opaque body, by the reaftion of the par- 

 ticles of light, muft be fomewhat moved 

 when the light is refled:ed backward and for- 

 ward between the fame particles, it is ma- 

 nifeft that they likeways mud be driven 

 backward and forward with a vibratory mo- 

 tion J and the time of a vibration will be e- 

 qual to that which light takes in moving 

 thro' a particle, or from one particle of a 

 body to another adjoining. This diftance in 

 moft folid opaque bodies cannot be fuppofed 

 greater than ttytt^^ of an inch, whicli 



fpace 



* I have found, by repeated trials, that the heat of wa- 

 fer in deep Jakes decreafes regularly from the furface down- 

 U'ardSf 



