1 68 SCIENTIFIC RECREATIONS. 



the different rate of the impingement of impulses on the retina. The 

 eyes, as we have already learned, cannot receive any more rapidly-recurring 

 impressions than those producing violet, although as proved, the spectrum 

 is by no means exhausted, even if they are invisible. In the consideration 

 of Calorescence we pointed this out. These invisible rays work great 

 chemical changes when they get beyond violet, and are shown to be heat. 

 So the rays which do not reach the velocity of red rays are also heat, which 

 is the effect of motion. 



Thus we have HEAT, LIGHT, and SOUND, all the ascertained results of 

 vibratory motion. The stillness of the ether around us is known as " Dark- 

 ness " ; the stillness of the air is " Silence " ; the comparative absence of 

 heat, or molecular motion of bodies is " Cold " ! 



In the first part we showed how coins impart motion to each other. 



When an impulse was given the motion was carried from coin to coin, and at 

 length the last one in the row flew out. This is the case with sound. The 

 air molecules strike one upon another, and the wave of " sound " reaches the 

 tympanum, and thus the impression is conveyed to the brain. We say we 

 hear but why we hear, in what manner the movement of certain particles 

 effects our consciousness, we cannot determine. 



That the air is absolutely necessary to enable us to hear can readily 

 be proved. The experiment has frequently been made ; place a bell under 

 the receiver of an air-pump, and we can hear it ring. But if we exhaust 

 the air the sound will get fainter and fainter. Similarly, as many of us have 

 "* experienced upon high mountains, sounds are less marked. Sound dimin- 

 ishes in its intensity, just as heat and light do. Sound is reflected and 

 refracted, as are light and radiant heat. We have already shown the effect 

 of reflectors upon heat. Sound is caught and reflected in the same way as 



