390 TRANSACTIONS OF SECTION A. 
Quay tube, and quite recently by Miss Hedwig Cohn for metal vapours in flames. So 
these processes are of the type of pure temperature radiation. : 
In order to bring this result into agreement with former experiments made by 
myself and by others, I put forward the hypothesis that in general two processes 
must work together for originating the spectral emission of a gas; by the first the 
centres of emission (electrons of dispersion) must be created, by the second these 
centres must be excited toemission. Hyen when—as it is in the case of pure tempera- 
ture radiation—the excitement is due to the collisions of the atoms containing a vibrator 
with other molecules or atoms, whose average kinetic energy is a function of tempera- 
ture only, the creation of the vibrators may be due in full or partially to processes of 
quite another nature; for instance, to chemical or electrical actions. 
In the case of luminescence we can follow a method quite analogous to that given by 
Planck in his theory of black radiation. In the state of thermodynamical equilibrium 
according to Planck the average energy of a resonator of the frequency v at the abso- 
lute temperature T is given by 
Se A sn toa 
where fi and BR are the well-known constants. ‘This average energy in the case of 
temperature radiation is given to the resonator by the permanent exchange between its 
own energy and the kinetic energy of the molecules. The only term in the formula 
connected with the energy of molecules is the term RT, which is proportional to the 
average energy of the exciting impulses. In the case of luminescence a stationary 
state of equilibrium can also take place in all the phenomena, where the emissivity 
E, and the absorption power A, are independent of the density of radiation. (Therefore 
our theory cannot be applied to the phenomena of phosphorescence and fluorescence.) 
When this state of equilibrium is established, the emission will arise by the interchange 
of energy between the exciting impulses and the vibrating resonators. Provided that 
all radiating resonators are excited by a similar mechanism following the same law, 
the average energy of a resonator will be given by 
T= een! hava de Sik Nan 
hy 
efO—]1 
where f (7) is a definite funetion of the intensity 7 of the process by which the lumines- 
cence is excited. For a body which follows the equation (1) according to Kirchhofi’s 
law we would have : 
When in equation (2) we put 
f(t) =RT, 
it takes the form of equation (1) for the average energy of a resonator in the black 
radiation of the temperature T,. In this way it follows that in the state of equilibrium 
characterised by equation (2) we must have : 
2 =@7T,- . e 5 e ° . e 5 (3). 
As A, and E, are presumed to be independent of the established state of radiation, 7 
A 
must be a quantity characteristic for the given phenomenon of luminescence, given by 
the emissivity ear, of a black body of the temperature T,, even when there is no state 
of equilibrium in the luminescent body. 
So we have to expect very near parallelism between the phenomena of lumines- 
cence and of temperature radiation, as is observed in many cases. The difference 
between these two classes of phenomena seems to be much smaller than we thought 
before.» On the other hand, when we find that in a phenomenon of luminescence the 
equation (3) leads to the same temperature T, for different spectral lines, we can con- 
clude that the mechanism of emission of different wave-lengths may be the same. sisi 
