THE TEACHINGS OF MODERN SPECTROSCOPY. 475 



If we find three spectra of nitrogen and the same number for oxygen, 

 we must accept the verdict, and conclude that these gases can exist in 

 three different allotropic states. 



Among the remarkable phenomena observed in vacuum-tubes, per- 

 haps not the least curious is the spectrum observed at the negative 

 pole, which in several cases is only observed there, and under ordinary 

 circumstances in no other part of the tube. Both oxygen and nitro- 

 gen have a spectrum which is generally confined to the negative glow. 

 Some years ago I tried to prove that also in these cases we have only 

 to deal with a special modification of the gases which, curiously enough, 

 only exists near the negative pole, and is broken up and decomposed in 

 every other part of the tube. The experiments I then made seem to 

 me to prove the point conclusively. After a current of electricity had 

 passed through the tube for some time in one direction, the current 

 was suddenly reversed ; the negative pole now became positive, but 

 the spectrum still was visible for some time in its neighborhood, and 

 only gradually disappeared. This experiment shows that the spectrum 

 may exist in other parts of the tube, and that it is therefore due to a 

 peculiar kind of molecule, and not to anything specially related to 

 electric phenomena taking place in the neighborhood of the negative 

 pole. Other experiments supported this view. 



The classification of spectra, according to the complexity of the 

 vibrating molecule, is of great theoretical importance ; for by its 

 means we may hope to obtain some information on the nature of the 

 forces which bind together the atoms into one molecule. Our whole 

 life is a chemical process, and a great part of the mysteries of Nature 

 would be cleared up if we could gain a deeper insight into the nature 

 of chemical forces. I believe no other line of investig:ation to be as 

 hopeful in this respect as the one which examines directly the vibra- 

 tions of the molecules which take place under the influence of these 

 chemical forces. If we could find a connection between the vibrations 

 of a compound molecule and the vibrations of the simpler elements 

 which it contains, we should have made a very decided step in the de- 

 sired direction. I need not say that various attempts have been made 

 to clear up so important a point ; but we have to deal with compli- 

 cated forces, and the attempts have as a rule not been crowned with 

 much success. 



There are, however, a few exceptions, a few cases of greater sim- 

 plicity than the rest, where we are able to trace to their mechanical 

 causes the spectroscopic changes which take place on chemical com- 

 bination. These few and simple cases may serve as the finger-posts 

 which show us the way to further research, and, we may hope, to 

 further success. To make the spectroscopic changes of which I am 

 speaking clear to you, I must have recourse to the analogy between 

 sound and light, and remind you of the fact that when the prongs of 

 a tuning-fork are weighted its tone is lowered, which means that the 



