LIGHT. 127 



porous bodies are opaque ; cork, charcoal, pumice stone, dried 

 and moist wood, &c., all very porous and very light, are all 

 opaque. This objection is not so superficial as it might seem 

 at first sight. The theory which assumes that light is an 

 undulation of an ethereal medium pervading gross matter, 

 assumes the distances between the molecules or atoms of 

 matter to be very great. Matter has been likened by Demo- 

 critus, and by many modern philosophers, to the starry firma- 

 ment, in which, though the individual monads are at immense 

 distances from each other, yet they have in the aggregate a 

 character of unity, and are firmly held by attraction in their 

 respective positions and at definite distances. Now, if matter 

 be built up of separate molecules, then, as far as our knowl- 

 edge extends, the lightest bodies would be those in which the 

 molecules are at the greatest distances, and those in which 

 any undulation of a pervading medium would be the least 

 interfered with by the separated particles such bodies should 

 consequently be the most transparent. 



If, again, the analogy of the starry firmament held good, 

 in this case an undulation or wave proportioned to the indivi- 

 dual monads would be broken up by the number of them, and 

 the very appearance of continuity which results, as in the 

 milky way, from each point of vision being occupied by one 

 of the monads, would show that at some portion of its pro- 

 gress the wave is interrupted by one of them, so that the 

 whole may be viewed in some respect as a sheet of ordinary 

 matter interposed in the ethereal expanse. 



Even then, if it be admitted that a highly elastic medium 

 pervades the interspaces, the separate masses as a whole must 

 exercise an important influence on the progress of the wave. 



Sound or vibrations of air meeting with a screen, or, as 

 it were, sponge of diffused particles, would be broken up and 

 dispersed by them ; but if they be sufficiently continuous to 

 take up the vibration and propagate it themselves, the sound 

 continues comparatively unimpaired. % 



