156 Prof. E. Wiedemann on the 



The sulphur compounds of the metals of the alkaline earths, 

 glowing in consequence of chemi-luminescence, yield light of 

 different colours according to difference of temperature. 



The order of intensity of emission of light may be com- 

 pletely reversed by change in the mode of excitation. Thus 

 if, in electro-luminescence, a body A shines as bright as, or 

 brighter than, another body B, this will not necessarily be the 

 case upon ignition or with chemi-luminescence. Mercury and 

 sodium give us examples of this. The first, introduced in 

 the gaseous state into the flame, gives scarcely any light, 

 whilst in a Geissler's tube it gives an intensely bright light ; 

 sodium, on the other hand, is very bright in both cases. 



10. In many cases luminescence and ignition occur together. 

 If we wish to arrive at conclusions in such cases we must 

 endeavour to separate the two phenomena. The following are 

 probably processes in which both phenomena occur together : — 



In flame the production of light depends partly upon chemi- 

 luminescence and partly upon phenomena of ignition, so soon 

 at least as solid particles are separated. 



If electric discharges pass between metal electrodes the 

 metal is disintegrated and volatilized, and the vapour is heated 

 to incandescence, at the same time it may be brought to 

 luminescence by the electric current. 



If we pass through a tube filled with hydrogen discharges 

 of such a strength that the line-spectrum just appears, the 

 hydrogen is far from being heated to the temperature of 

 incandescence. If, on the other hand, we employ very strong- 

 discharges, we have, in addition to the original luminosity, a 

 very great rise in temperature, which produces incandescence. 

 We can expect to obtain an absorption corresponding to 

 Kirchhoff's law only for the rays emitted by the process of 

 incandescence, but not for the others. 



The phenomena of long and short lines observed when an 

 image of a horizontal flame is thrown upon the vertical slit 

 of a spectroscope depend, no doubt, in part at least, upon 

 the different processes of luminescence and incandescence. 

 Several factors are simultaneously concerned in their pro- 

 duction. Thus, for example, the short lines appear in the 

 inner portions, and the long lines both in the inner and outer 

 portions of the flame. But in the inner portions, according 

 to the usual arrangement of the experiment, both the tempe- 

 rature and the quantity of incandescent material are greater 

 than in the external portion. Experiments are in progress to 

 separate these different conditions ; this is especially important 

 in order to decide which lines in the spectrum are produced 

 by luminescence and which by incandescence ; and how these 



