BY COLOURED MEDIA. 491 



nature than mere juxtaposition), constitute a distinct 

 compound vibrating system, in which parts differently 

 elastic are intimately united and made to influence each 

 other's motions. Of such systems in acoustics we have 

 no want of examples in membranes stretched on rigid 

 frames, in cavities stuffed with fibrous or pulverulent 

 substances, in mixed gases, or in systems of elastic 

 laminae, such as boards, sheets of glass, reeds, tuning 

 forks, &c., each having a distinct pitch of its own, and 

 all connected by some common bond of union. In all 

 such systems the whole will be maintained in forced 

 vibration so long as the exciting cause continues in 

 action, but the several constituents, regarded separately, 

 will assume, under that influence, widely different ampli- 

 tudes of oscillation, those assuming the greatest whose 

 pitch taken singly is nearest to coincidence with that of 

 the exciting vibrations. Everybody is familiar with the 

 tremor which some particular board in a floor will 

 assume at the sound of some particular note of an organ ; 

 but when that note is not sounded, it is sufficiently 

 apparent that the board is no less occupied in perform- 

 ing its dynamical office of transmitting to the soil below, 

 or dispersing through its own substance and the con- 

 tiguous bodies, the motion which the oscillation of the 

 air above is continually imparting to it. 



(17.) As we know nothing of the actual forms and 

 intimate nature of the gross molecules of material bodies, 

 it is open to us to assume the existence, in one and the 

 same medium, of any variety of them which may suit 

 the explanation of phenomena. There is no necessity 



