( 594 ) 



The equations may be fiirtlier simplitied, if, following a well 

 known method, we work with complex expressions, all containing 

 the time in the factor e«»'. If we introduce the three quantities 



g=/' — a — in li*, (11) 



il^ng,. (12) 



and 



«H 



^=im'- "'' 



the result becomes 



^, = (t-Yin)\\ + i^%. (14) 



(T-, = (§ + ; li) %. ' 



§ 5. Before proceeding further, we shall try to form an idea of 

 the mechanism by which the absorption is produced. It seems difficult 

 to admit the real existence of a resistance proportional to the velo- 

 city such as is represented by the expressions (8). It is true that in the 

 theory of electrons a charged particle moving through the aether 

 is acted on by a certain force to which the name of resistance may 

 be applied, but this force is proportional to the differential coefficients 

 of the third order of x, y, z with respect to the time. Besides, as 

 we shall see later on, it is much too small to account for the absorp- 

 tion existing in many cases; we shall therefore begin by neglecting 

 it altogether, i. e. by supposing that a vi!)rating electron is not subject 

 to any force, exerted by the aether and tending to ilanip its vibrations. 



However, if, in our case of gaseous bodies, we think of the mutual 

 encounters between the molecules, a way in which the regular 

 vibrations of light might be transformed into an inorderly motion 

 that may be called heat, can easily be conceived. As long as a mole- 

 cule is not struck by another, the movable electron contained within 

 it may be considered as free to follow the periodic electric forces 

 existing in the beam of light ; it will therefore take a motion whose 

 amplitude would continually increase if the frequency of the incident 

 light corresponded exactly to that of the free vibrations of the electron. 



In a short time however, the molecule will strike against another 

 particle, and it seems natural to suppose that by this encounter the 

 regular vibration set up in the molecule will be changed into a 

 motion of a wholly different kind. Between this transformation and 

 the next encounter, there will again be an interval of time during 

 which a new regular vibration is given to the electron. It is clear 

 that ill this way, as well as Ity a resistance proportional to the velo- 



