10 SCIENCE 1$ SHORT CHAFFERS. 



forward, this push to be communicated to the next, then to 

 the third, and so on, producing a minute running tremor 

 passing from end to end. This kind of action may be rendered 

 visible by laying a number of billiard balls or marbles in line 

 and bowling an outside ball against the end one of the row. 

 The impulse will be rapidly and invisibly transmitted all along 

 the line, and the outer ball will respond by starting forward. 



Heat, light, and electricity are mysterious internal move- 

 ments of what we call matter (some say ** ether," which is but 

 a name for imaginary matter. ) These internal movements are 

 as invisible as those of the intermediate billiard balls ; but if 

 there be a line of molecules acting thus, and the terminal one 

 strikes an organ of sense fitted to receive its motion, some sort 

 of perception may follow. When such movements of certain 

 frequency and amplitude strike our organs of vision, the sensa- 

 tion of light is produced. When others of greater amplitude 

 and smaller frequency strike the terminal outspread of our 

 common sensory nerves, the sensation of heat results. The 

 difference between the frequency and amplitude of the heat 

 waves and the light waves is but small, or, strictly speaking, 

 there is no actual line of separation lying between them ; 

 they run directly into each other. When a piece of metal is 

 gradually heated, it is first " black-hot ;" this is while the 

 waves or molecular tremblings are of a certain amplitude and 

 frequency ; as the frequency increases and amplitude dimin- 

 ishes (or, to borrow from musical terms, as the pitch rises), 

 the metal becomes dull red-hot ; greater rapidity, cherry red ; 

 greater still, bright red ; then yellow-hot and white-hot : the 

 luminosity growing as the rapidity of molecular vibration 

 increases. 



There is no such gradation between the most rapid undula- 

 tions or tremblings that produce our sensation of sound and 

 the slowest of those which give rise to our sensations of 

 gentlest warmth. There is a huge gap between them, wide 

 enough to include another world or several other worlds of 

 motion, all lying between our world of sounds and our world 

 of heat and light, and there is no good reason whatever for 

 supposing that matter is incapable of such intermediate activity, 

 or that such activity may not give rise to intermediate sensa- 

 tions, provided there are organs for taking up and sensifying 

 (if I may coin a desirable word) these movements. 



As already stated, the limit of audible tremors is three to 

 four thousand per second, but the smallest number of tremors 



