NATURAL PIIILOSOrHT. 131 



violent stir has been given to the fuel, and a torrent of flarae sent 

 throijfrh the flues; and we rather think explosions have really 

 occurred under such circumstances. — Mechanics' Magazine. 



MOLECULAR IKFLUEN'CE UPON LIGHT. 



At the closing meetinnr of the Roval Institution, Dr. Frankland 

 brought forward some new experiments upon light, of a very 

 interesting character, proving that external pressure, acting upon 

 the ponderable molecules, which throw out waves of light, exerts 

 considerable influence in modifying the character of the resulting 

 luminosity. The exceedingly minute ultimate atoms, composing 

 what is called solid matter, utterly beyond the ken of the most 

 powerful microscope, are believed by Messrs. Stewart, Tyndall, 

 Thompson, and other i>hilosophers of the same high school, to be 

 in a state of incessant motion, of a true or modified vibratory 

 character. This motion is what is commonly called heat. The less 

 the vibratory motion of the solid, the lower is the temperature. 

 There is a point at which atomic motion would cease ; but such a 

 degree of cold has never yet been produced by man. 



The atoms in moving throw out waves. A fluid infinitely finer 

 than air, the "interstellar ether," is believed to fill all space, and 

 to permeate all solid bodies, bathing the vibratory particles. The 

 explanations, given by philosophers, of the causes of all the phe- 

 nomena of optics and the undulatory theory of light, rest upon 

 the assumption of the reality of this ether. The atoms, then, can- 

 not move without throwing the ether into waves ; neither can the 

 ether move without altenng the motion of the atoms. The waves 

 thrown off by moving atoms differ in length ; most of the shorter 

 waves being what is known as light, and the longer waves being 

 called radiant heat. Hence there is no difference between radi- 

 ant heat and radiant light but that of '* wave-length." That our 

 senses convey to us the impression that there is a very great dif- 

 ference between heat and light is caused by the shoit range of 

 action of our senses, not by any gi-eat difference in the phenom- 

 ena. The hand is sensitive to the long, slow waves of heat, but is 

 practically insensitive to the quick, short waves of light. The eye 

 is very sensitive to the short, quick waves, which we call light, 

 but, according to Tyndall, is very insensitive to radiant heat. 

 Thus most of the waves emitted by all ordinary sources of light 

 are invisible, as may be proved by throwing the spectrum of the 

 electric light upon a screen. In the spectrum, all the rays in light 

 are unrolled, so to speak, in regular order, the longest waves at 

 one end and the shortest at the other. The shorter waves, at one 

 end, are visible to us, beginning with the violet and ending with 

 red, the last being the longest and slowest which can be seen by 

 the eye. But when the red is reached, we have only reached the 

 middle of the spectrum; beyond the red, another length of sepa- 

 rated rays falls upon the screen, invisible to the eye. That there 

 are ravs there, we know from the delicate little thermo-electrom- 

 eter, which proves that the spectrum of the electiic light is 



