PHILOSOPHICAL SOCIETY OP WASHINGTON. 6T 



thus learn that for the mean wave-length of light (about the 45 

 thousandth of an inch), there are about 500 billion of these 

 atomic oscillations or revolutions in one second ; so that, brief as 

 is the interval of molecular excursion from neighbor to neighbor, 

 about 83 thousand of these undisturbed internal pulsations occur 

 in the interval. There is thus a constant exchange and tendency 

 to equalization of vis-viva between the atomic vibrations, and the 

 molecular flights producing them ; and from the very nature of 

 the mode of transfer by successive irregular or variable impacts, 

 we are forced to conceive these atomic motions as variable ellip- 

 •ticul orbits, the straight line and the circle being the maximum 

 and ininimum limits of elliptic eccentricity. The radiation of 

 heat is always of atomic origin, and is transmitted with the ve- 

 locity of liglit. Tlie conduction of heat is always of molecular 

 origin, and is communicated through material contacts or impacts, 

 and with very great slowness, from the time required to diffuse 

 motions with such extreme irregularity and prevalent obliquity 

 of impacts. 



Knowing the mean period of the atomic orbits, if the average 

 velocity communicated to them at collisions equalled the average 

 velocity of the molecular flights, the average magnitude of the 

 orbit might easily be computed at about the 25,000 millionth 

 part of an inch, or the 500 thousandth of a mean wave-length. 

 But there is reason to believe that the average velocity is much 

 less than 1640 feet per second, and the mean diameter of the 

 orbit correspondingly smaller; since of the total or aggregate 

 vis-viva — external and internal — of the molecules, constituting 

 the specific heat of a gas, more than half belongs to the trans- 

 latory motion. That the orbit constituting the width of the wave 

 must be exceedingly small, is evident from the very wide range 

 of variation to which it is subject within the molecular magnitude. 

 The large luminous orbits undoubtedly exceed the smallest by 

 hundreds of thousands of times, or probably as sixteen miles ex- 

 ceeds one inch. The amplitude of the v/aves is as the square 

 root of the intensity (which may vary a billion times), and in- 

 versely as the distance of the origin. 



The conclusion arrived at respecting the two unknown elements 

 of the luminiferous wave motion is, that the form is probably aa 

 elliptical orbit (as in the case of liquid waves), and that its diam- 

 eter — constituting the amplitude of the wave — is for light of ordi- 

 nary brilliancy, probably of an order approaching the billionth 

 of an inch. 



