620 Prof. E. Goldstein on Three/old 



The chief-spectrum is for a great number of substances so 

 characteristic that it is possible to recognize the substance in 

 this way at a glance and without measuring the wave-lengths, 

 just as you can recognize nitrogen by its well-known bands, 

 or hydrogen, mercury and helium by their line-spectra. 

 This is even the case with isomeric substances ; for one is 

 able to distinguish at a glance, for instance, the three isomeric 

 xylenes or other isomeric aromatic hydrocarbons. The third 

 kind of spectra, which is quite different from the two others, 

 appears if an aromatic substance is dissolved in any other 

 liquid or melted compound and the solidified solution is 

 exposed to cathode-rays. 



Now let me just say a few words on the properties of 

 each of the three kinds of spectra. 



The chief-spectra always begin from the infra-red, never reach 

 the violet end of the visible spectrum, but end about the middle 

 part of it in the green or in the blue, sometimes even in the 

 yellow. I never observed that a chief- spectrum passes the 

 wave-length of X 460. The chief-spectra consist of narrow 

 channelled bands, which nearly always have their sharper 

 boundary toward the violet end of the spectrum. The 

 number of the bands varies within a wide range for the 

 different substances between a few strips and several dozen. 

 The distances between them appear generally irregular. 

 The substances, when they send out their chief-spectrum, 

 look red or yellow or green, or of any other tint which 

 occurs with fluorescent minerals or inorganic salts. On the 

 other hand, the light which is emitted during the first 

 moments of radiation and belongs to the initial-spectrum is 

 — at least, for colourless substances — ahvays blue. The dis- 

 continuous initial-spectra of two substances are, like their 

 chief-spectra, never quite the same ; but as in their general 

 appearance they are rather similar to each other, so one cannot 

 recognize a substance at a glance by its initial-spectrum as one 

 can by the chief-spectrum, but measures of its wave-lengths 

 are necessary. The initial-spectra begin always like the 

 chief-spectra in the red ; but not only reach into the green 

 or blue, but go on into the ultra violet. One type of 

 initial-spectra occurring especially frequently invariably 

 consists of six groups of bands. Each of the six groups 

 is formed by the same number of strips at the same 

 relative distance and intensity ; and as the relative distance 

 of the groups themselves is also not very different — at least 

 in the prismatic spectrum — the whole spectrum gives the 

 impression of having a very high regularity. Such spectra, 

 consisting of six groups, with different wave-lengths for each 



