264 Dr. A. Schuster on the Dynamical 



tion in the enclosure is very nearly the same as that of the 

 flame radiating into space; and consequently a removal of the 

 flame into the enclosure will not materially affect the average 

 energy of vibration for yellow light. But the internal radia- 

 tion for the green light being much stronger in the enclosure 

 of uniform temperature, the energy of vibration of the medium 

 within the flame will be much increased by the removal of 

 the flame; and this cannot take place without a corresponding 

 increase in the energy of vibration of the molecules them- 

 selves. It follows from this that the difference in the inten- 

 sity of the yellow and green vibration of sodium must be 

 much smaller in the enclosure of uniform temperature than 

 when the sodium-flame is allowed to radiate into space, though 

 the temperature in both cases may be the same. 



The following suggestion as to the genesis of radiation is 

 founded on the ideas of the molecular theory of gases, and 

 may serve to make the difference between the molecular vi- 

 brations in the case of a radiation into a colder space as dis- 

 tinct from a radiation into an enclosure of equal temperature 

 a little more clear. In a gas which is neither gaining nor 

 losing heat we assume a constant interchange of motion 

 between different molecules. 



Let us assume that the molecules we are considering are 

 like the sodium molecules, capable of vibrating in two distinct 

 periods, one corresponding to the yellow and one to the green 

 rays. The energy of vibration for each molecule is constantly 

 changing ; the translatory energy of motion is, by means of the 

 encounters, transformed into vibratory energy corresponding 

 to one or to the other period; and if, on the whole, the average 

 energy remains constant, it is because just as much vibratory 

 energy is transformed into energy of translatory motion. We 

 may imagine that this energy of translatory motion is more 

 easily transformed into yellow vibrations than into green vibra- 

 tions; but in that case the energy of yellow vibrations must 

 also more easily be transformed into translatory motion. The 

 mathematical calculations of Maxwell and Boltzmann have 

 shown that when the final equilibrium has been reached the final 

 energy of vibration for the two periods must be the same, 

 though each encounter may more easily produce vibrations of 

 one period than of the other. This theoretical conclusion is 

 not altogether confirmed by experience; but we may, for the 

 sake of argument, assume it to hold in the imaginary case 

 which we are at present considering. Imagine now the gas 

 to be taken out of the enclosure and allowed to radiate into 

 space. During the first instant the yellow and the green 

 vibrations will be of the same intensity; but very soon the 



