OF COMMON MATTER. 161 



Obstruction. It is further proved by the utter 

 impossibility which exists, of exhausting- the 

 solar rays out of a glass receiver, after the other 

 parts of the atmospheric materials have been 

 pumped out of it. 



Such, indeed, is the extreme subtilty of the 

 solar rays, that their materiality has been ques- 

 tioned. I do not mean to range myself under 

 the banners of those who entertain these doubts : 

 to me it appears, that the materiality of light is 

 as decisively proved, as the materiality of any 

 species of matter whatever. The impressions 

 excited upon the different organs of sense by 

 external objects, of ice, or of fire, in producing 

 the sensation of cold or of heat of air upon 

 the auditory nerves, in exciting the sensation of 

 sound of volatile bodies upon the olfactory 

 nerves, in exciting the sensation of odor, &c. &c. 

 are not more evident and characterised, than 

 the power which the solar rays possess, of pene- 

 trating the cornea, or external coat of the eye ; 

 and, after undergoing different modifications, 

 from the different media through which they 

 pass, of arriving at the bottom of the eye, on 

 which the retina is spread, sensation excited, 

 and illumination produced. The materiality 

 of the solar rays is proved, by the space which 

 they fill, and by the illumination which they 

 excite. It is proved by the facility with whick 



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