PHYSICAL anp LITERARY. 23 
principles, fome imperfect conception of - 
the manner in which bodies are heated 
and’ burnt by the aétion of light: More 
than an imperfect notion of thefe fecret 
operations of nature is not to be expect- 
ed; for they certainly depend, in great 
meafure, upon laws and principles utterly 
unknown to us. 
_ 13. Ie one beam or ray of light, by paf- 
fing ftraight onwards through the fame 
pellucid fubftance, can communicate no 
heat to its internal parts, neither will the 
greateft quantity of rays, though crowded 
into the narroweft {pace, by crofling one 
another. From hence it follows, that the 
portion of air which lies in the focus of the 
mott potent /peculum is not at all affected - 
by the paffage of light through it, but 
continues of the fame temperature with 
the ambient air; although any opaque 
body, or even any tran{parent body den- 
fer than air; when put in the fame 
place, would be intenfély heated in an in- 
flant. 
14, THs confequence, evidently flow- 
_ ing from the plaineft and moft certain 
princi: 
