482 ON THE ABSORPTION OF LIGHT 



in a medium consisting of loosely-aggregated earth inter- 

 mixed with much air, the hollow sounds which are often 

 attributed to the reverberation of subterrannean cavities, 

 and in particular the celebrated instance of this kind of 

 sound heard at the Solfaterra near Pozzuoli. The dull 

 and ill-defined sound thus produced from a succession of 

 partial echoes is there assimilated to the nebulous light 

 which illuminates a milky medium when a strong beam 

 is intromitted. If we suppose, now, such a mass of 

 materials insulated from communication with the exter- 

 nal air by some sound-tight envelope, these partial echoes, 

 when they reach the surface in any direction, will be all 

 sent back again as so many fresh impulses, till at length 

 it will become impossible to assign a point within the 

 mass which will not be agitated at one and the same 

 moment by undulations traversing it in every possible 

 phase and direction. Now the state of a molecule, un- 

 der the influence of an infinite number of contradictory 

 impulses thus superposed, is undistinguishable from a 

 state of rest. 



(8.) The only difficulty, then, which remains in the 

 application of the undulatory theory to the absorptive 

 phaenomena, is to conceive how a medium (i.e., a combin- 

 ation of sethereal and gross* molecules) can be constituted 

 so as to be transparent, or freely permeable to one ray 

 or system of undulations, and opake, or difficultly per- 



* By gross molecules, or gross bodies, I understand the ponderable 

 constituents of the material world, whether solid, liquid, or gaseous ; 

 using the term in contradistinction to zethereal, which has refer- 

 ence to the luminiferous aether. 



