10 Light and Sound. LJan., 



density. Wood transmits sound perfectly well, so does air, but 

 if the wood be reduced to sawdust, sound is no longer transmitted. 

 Thus cotton-wool deadens sound. Solid glass, salt, and ice, and 

 all transparent solids^ transmit light and sound ; reduced to powder, 

 they are opaque both to sound and light. A glass of water trans- 

 mits both light and sound, so does carbonic acid gas; but if the 

 two be mixed, as in soda-water, a mixture nearly opaque to both 

 light and sound is the result. By tapping a glass of soda-water 

 during and after the effervescence the sound effect can readily be 

 heard. The bubbles of gas break up the continuity of the water 

 and render the sound emitted by the glass dull ; when the bubbles 

 have escaped, the ringing note of the glass returns. The same 

 effect occurs in gaseous media. Humboldt, when journeying upon 

 the plains of the Orinoco, noticed that columns of alternately hot 

 and cold air enfeebled the passage of sound just as they lessened 

 the passage of light. In the same way through fog, sound, like 

 light, is propagated with extreme difficulty. The cause of this 

 arises from the particles of water that are then suspended in the 

 atmosphere, and by their presence produce variations of density. 



It has too been generally noticed that a good hearing day pre- 

 cedes wet weather, and before a wet day we generally notice the 

 atmosphere becomes remarkably clear. This clearness has by an 

 eminent physicist * been attributed to the fact that on ordinary 

 days our atmosphere contains innumerable spores or germs floating 

 in the air. At the approach of wet weather the little germs absorb 

 the moisture, they become heavier and sink to the ground ; thus 

 the atmosphere is cleared from these solid particles which at the 

 same time obstructed the free transmission of light, and interfered 

 with the free transmission of sound. 



§ 5. Absorption of Light and Sound. 



A black glass, or a black surface like velvet, absorbs the light 

 that falls upon it. The light, as already observed, becomes trans- 

 lated into heat in the absorbing body. Woollen textures, curtains, 

 and the like, are to sound what a dark glass is to light. Such 

 obstacles stop the progress of the sonorous pulses, and also convert 

 what was sound into heat. In the case of light there is further a 

 selective absorption of certain rays ; this gives rise to the colour of 

 a body. Such selective absorption is a molecular act, probably due 

 to the synchronism between certain luminous waves and the oscilla- 

 tions of the particles in their path. Hence selective absorption can 

 only exist when there is a mixture of waves from which to select. 

 Transferring these ideas from particles to a body as a whole, a 

 corresponding selective absorption exists when there is a complex 



* De La Rive. 



