462 tECTURE XXXIX. 



The chemical process of combustion may easily be imagined either to dis- 

 engage the particles of light from their various combinations, or to agitate 

 the elastic medium by the intestine motions attending it: but the operation 

 of friction upon substances incapable of undergoing chemical changes, as 

 well as the motions of the electric fluid through imperfect conductors, afford 

 instances of the production of light in which there seems to be no easy way 

 of supposing a decomposition of any kind. The phenomena of solar phos- 

 phori appear to resemble greatly the s^'mpathctic sounds of musical instru- 

 ments, which are agitated by other sounds conveyed to them tbrough the 

 air: it is difficult to understand in wliat state the corpuscles of light could 

 be retained by these substances so as to be reemittcd after a short space or 

 time; and if it is true that diamonds are often found, which exhibit a red 

 light after having received a violet light only, it seems impossible to explain 

 this property, on the supposition of the retention and subsequent emission 

 of the same corpuscles. 



The phenomena of the aberration of light agree perfectly well with the 

 system of emanation ; and if the ethereal medium, supposed to pervade the 

 earth and its atmosphere, were carried along befoie it, and partook materia^y 

 in its motions, these phenomena could not easily be reconciled with the 

 theory of undulation. But there is no kind of necessity for such a supposi- 

 tion: it will not be denied by the advocates of the Newtonian opinion that 

 all material bodies are sufficiently porous to leave a medium pervading them 

 almost absolutely at rest; and if this be granted, the effects of aberration 

 will appear to be precisely the same in either hypothesis. 



The unusual refraction of the Iceland spar has been most accurately and 

 satisfactorily explained by Iluygens, on the simple supposition that this 

 crystal possesses the property of transmitting an impulse more rapidly in one 

 direction than in another; whence he infers that the undulations constilutintr 

 light must assume a spheroidical instead of a spherical form, and lays down such 

 laws for the direction of its motion, as are incomparably more consistent 

 with experiment than any attempts which have been made to accoiiimndate 

 the phenomena to other principles. It is true that nothing has yet been 

 done to assist us in understanding the effects of a subsco" nt refraction by 

 a second crystal, unless any person can be satisfied with the name of polarity 



