1870.] Light and Sound. 3 



nous." Hero, then, we Lave at once the explanation, and the ana- 

 logy of the mechanical origin of luminous and sonorous vibrations. 



It is possible, however, to produce sound as well as light by a 

 molecular motion of vibration. This is the case when a wire or 

 rod is rubbed longitudinally ; and, as might be predicted, the sound 

 thus obtained is far more shrill than when the same body vibrates 

 transversely. Here, indeed, in longitudinal vibration, there are some 

 striking points of contact between light and sound. For if a beam 

 of polarized light be transmitted through a strip of glass, as soon 

 as the glass, by rubbing it longitudinally, is caused to emit a sound, 

 the light is powerfully affected. 



Light and sound have, further, a common origin in molecular 

 change when they are generated by chemical action ; thus light and 

 more or less of sound attends combustion ; and, directly or indirectly, 

 sound and more or less of light attend the explosion of fulminat- 

 ing powders. Again, light is produced by electricity, as in lightning 

 or the electric spark and electric light, and sound simultaneously 

 accompanies each of these phenomena. Light attends the quick 

 evolution of heat, and sudden heat is productive of the loudest 

 sounds, as in the explosion of mixed oxygen and hydrogen gases. 



Now, as light and sound are both the products of motion, the 

 law of the conservation of force teaches us that neither one nor the 

 other can have been produced without the loss of an equivalent 

 amount of motion, that is force, elsewhere; and, moreover, that 

 neither can have disappeared without the production of an equiva- 

 lent motion or force of another kind. This being so, the doctrine 

 of the correlation, or mutual convertibility, of the physical forces 

 comes in and shows us the possibility, not at once perhaps but 

 through intermediate steps, of exchanging light for sound and sound 

 for light. As it is, already we know that the quenching of both 

 light and sound, by absorbing media, results in the production of the 

 same mode of motion, namely, that which we designate heat. 



§ 2. The Pkopagation of Light and Sound. 



Originating in vibratory motion, let us now inquire in what 

 manner the forces of light and sound reach the eye and ear respec- 

 tively. When a stick is allowed to swing to and fro in still water, 

 the motion is communicated to the medium around, and a series of 

 waves travel outward from the centre of disturbance. The motion, 

 or vis viva, of the stick, though retarded and finally brought to rest 

 by the friction of the water, is not lost. The movement has been 

 delivered to the water, and part of it has reappeared in the form of 

 the waves we noticed. The same considerations apply to light and 

 sound. When a bell is struck its vibrations are delivered to the 

 air around ; a system of aerial pulses or waves is thus generated, 



b 2 



