ii 4 ELEMENTS OF SCIENCE 



here noted as to the resemblance between light and heat) 

 analogous phenomena with respect to heat,* show that 

 heat rays have also their polarity. When we come to 

 consider " magnetism," we shall meet with another kind 

 of polarity, which is conspicuously manifested and very 

 remarkable in its effects. 



SOUND. As the recurrence of certain actions, or 

 influences, which take place in bodies, produces in us 

 perceptions of heat or light, so the occurrence, or 

 recurrence, of certain motions of another kind, produces 

 in us perceptions of sound. 



If two solid bodies are struck together, the shock of 

 their contact gives rise to a sound, and certain bodies 

 are distinguished as sonorous because very slight 

 impulses will cause them to give forth very perceptible 

 sounds, as, for example, the strings of a fiddle or the 

 metal of a gong. 



An impulse given to any body surrounded by air. is 

 necessarily imparted by it to the immediately adjacent 

 portion of that aeriform fluid. This, owing to the 

 elasticity of air, rebounds after transmitting an impulse 

 to the next portion of air and so on the impulse being 

 transmitted by waves through the air in all directions 

 from the first starting-point. These aerial waves, or 

 oscillations, may be compared with the waves which 

 pass over the surface of a field of wheat when agitated by 

 wind, and they thus pass along (at a temperature of 62) 

 at a rate of 1125 feet in a second. The air of course does 

 not thus pass along (any more than the wheat does), but 

 only the waves of motion traversing it. The particles 

 of air, after each displacement, return to their former 



* See ante, p. 101. 



