LIGHT. 113 



consequently be the most transparent. It may be argued 

 that the size and want of continuity of the particles in porous 

 bodies arrests or disperses the undulations, but this argument 

 would apply on the ethereal hypothesis to all bodies. 



If the analogy of the starry firmament held good, in this 

 case an undulation or wave proportioned to the individual 

 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 progress 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. 



With regard, however, to liquid and gaseous bodies, there 

 are very great difficulties in viewing them as consisting of 

 separate and distant molecules. If, for instance; we assume 

 with Young that the particles in water are comparatively as 

 distant from each other as 100 men would be if dispersed at 

 equal distances over the surface of England, the distance of 

 these particles, when the water is expanded into steam, would 

 be increased more than forty times, so that the 100 men would 

 be reduced to two, and by further increasing the temperature 

 this distance may be indefinitely increased ; adding to the 

 effects of temperature, rarefaction by the air-pump, we may 

 again increase the distance, so that, if we assume any original 

 distance, we ought, by expansion, to increase it to a point at 

 which the distance between molecule and molecule should 

 become measurable. But no extent of rarefaction, whether 

 by heat or the air-pump, or both, makes the slightest change 

 in the apparent continuity of matter ; and gases, I find, retain 



I 



