410 On Light. 



verse, rare and elastic in a high degree. '* Is not the heat (of 

 the warm room) conveyed through the vacuum by the vibrations 

 of a much subtiler medium than air ? And is not this medium 

 the same with that medium by wliich light is reflected and re- 

 fracted, and by whose vibrations light communicates heat to 

 bodies, and is put into fits of easy reflection and easy transmis- 

 sion ? And do not the vibrations of this medium in hot bodies 

 contribute to the intenseness and duration of their heat ? And 

 do not hot bodies communicate their heat to contiguous cold 

 ones, by the vibrations of this medium, propagated from tliom 

 into the cold ones ? And is not this medium exceedingly more 

 rare and subtile than the air, and exceedingly more elastic and 

 active ? And doth it not readily pervade all bodies ? And is it 

 not by its elastic force expanded through all the heavens ?" — 

 " If any one would ask how a medium can be so rare, let him 

 tell me how an electric body can by friction emit an exhalation 

 so rare and subtile, and yet so potent ? and how the effluvia of 

 a magnet can pass through a plate of glass without resistance, 

 and yet turn a magnetic needle beyond the glass?" — Optics, Qu. 

 18, 22. " Were I to assume an hypothesis, it should be this, if 

 propounded more generally, so as not to determine what light is, 

 further than that it is something or other capable of exciting vi- 

 brations in the ether ; for thus it will become so general, and 

 comprehensive of other hypotheses, as to leave little room for new 

 ones to be invented." — Birch, iii. 249. 



Dr. Young shows, that many pha;nomena inexplicable on the 

 notion of radiating corpuscles, are easily reconciled to the theory 

 of undulation. *' On the whole," says this profound philosopher, 

 " it appears that the few optical phaenomena, which admit of ex- 

 planation by the corpuscular system, are equally consistent with 

 this theory; that many others which have been long known, but 

 never understood, become by these means perfectly intelligible ; 

 and th.at several new facts are found to be thus, only, reducible to 

 a perfect analogy with otlier facts, and to the simple principles 

 of the undulatory system." — Nat. Phil. vol. ii. p. 631. 



I think, however, that the new discoveries on polarized light 

 may be more easily referred to the corpuscular than undulatory 

 hypothesis. 



The physical affections of light are foreign to this work. Its 

 chemical relations are alone to be considered. These may be 

 conveniently referred to four heads : 



1. Of the mean refractive and dispersive powers of different 

 bodies. 



2. Of the action of the different prismatic colours on chemical 

 matter. 



3. Of the polarization of light. 



4. Of 



