ANOTHER WORLD DOWN HERE. 45 



strike the terminal outspread of our common sensory 

 nerves, the sensation of heat results. The difference be- 

 tween 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 gradu- 

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

 or molecular tremblings are of a certain amplitude and fre- 

 quency ; 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 undu- 

 lations 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 rea- 

 son whatever for supposing that matter is incapable of such 

 intermediate activity, or that such activity may not give 

 rise to intermediate sensations, 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 that we can perceive as heat is between three and 

 four millions of millions per second. The number of 

 waves producing red light is estimated at four hundred 

 and seventy-four millions of millions per second ; and for 

 the production of violet light, six hundred and ninety-nine 

 millions of millions. These are the received conclusions 

 of our best mathematicians, which I repeat on their au- 

 thority. Allowing, however, a very large margin of possi- 

 ble error, the world of possible sensations lying between 

 those produced by a few thousands of waves and any num- 

 ber of millions is of enormous width. 



In such a world of intermediate activities the insect 



