1870.] Light and Sound. 5 



two luminous undulations thus related. In the same way one 

 sound-pulse may obliterate another, and two sounds thus result in 

 silence. If, for example, a vibrating tuning-fork, placed near the 

 ear, be slowly turned on its axis, four times in each revolution, a 

 position will be reached where no sound is heard. This occurs 

 when the prongs are oblique in reference to the ear, and the waves 

 generated by each prong exactly interfere. If the interference be only 

 partial, beats, or intervals of sound and silence, will be produced. 



Eeturning now to the sounding-bell : its motion chiefly spends 

 itself on the air. Hence, when we remove the air from around 

 a sounding body, the vibration continues far longer than other- 

 wise it would. In such a case no sound is heard : internal friction 

 finally brings the sonorous body to rest as a whole, and molecular 

 motion or heat is the result. But in removing the air we do not, 

 and we cannot in any way as yet known, remove the luminiferous 

 ether ; by which ether heat is propagated as well as light. Hence the 

 heat-motion resulting from what originally was a sounding body at 

 last escapes from the enclosure in the form of ethereal undulations. 



Now when heat spreads by the ether in this way, we denominate 

 it radiant heat ; and, as we might expect, all the laws common to 

 light are also common to radiant heat, for both are undulatory 

 motions of the selfsame ether.* But whilst the propagation of 

 light only takes place (so far as we yet know) by means of this 

 ether, heat, on the contrary, spreads itself in two ways. It may 

 be.propagated from particle to particle by tangible matter, and the 

 phenomenon is then termed conduction ; or in waves by the ether, 

 and the phenomenon is then, as we have seen, termed radiant heat. 

 Now sound, like heat, may also be propagated by gross matter, and 

 this likewise is called conduction ; it may further, as already men- 

 tioned, spread itself in waves by the air, and in this form we may 

 conveniently call it radiant sound. Light being only known in the 

 radiant form, obviously it will only be in the phenomena of radiant 

 sound that we shall be justified in further seeking the analogy to 

 light. This we shall do in a succeeding section. 



Here, however, it is important to note that we might expect 

 an analogy between the phenomena of sound-conduction and heat- 

 conduction. This is the case : taken in connection with the foot- 

 note below, considerable support is thus given to the analogy of 

 sound and light. Broadly viewed, the best conductors of sound 

 are the best conductors of heat, and vice versa. Conduction in 

 both cases is best through solids and worst through gases. Metals 

 are the best conductors of sound and also of heat. In one very 



* The selective absorption of radiant heat by various media (thermocrosis), 

 the phenomena of calorescence — or the conversion of radiant heat into light — 

 and the warming of a black surface by purely luminous rays, demonstrate the fact 

 that radiant heat is only another phase of light. 



