

Temperature Radiation of Gases. 269 



stimulus, this, by preference, passes through the more easily 

 excitable impurities, leaving the He atoms unaffected. Franck 

 (Zeitsehrift jilr Phi/sik, 1920, vol.i.) describes an interesting 

 experiment on the excitation, by his well-known electron- 

 bombardment method, of helium-lines contained in a tube into 

 which a trace of mercury (less than 1 in 1000 parts) was 

 purposely introduced. Mercury has an I. P. of 10'45 volts 

 and R.P. of 4*9 volts, while the corresponding numbers for 

 helium are 25"6 and 20'5. " We have then," in Franck's 

 own words, " a strong flashing out of the lines of the element 

 with the lower I. P., at the cost of the lines with the larger LP." 

 This also explains qualitatively why, with the range of tempe- 

 ratures available in the laboratory, it is not possible by purely 

 thermal means to excite the permanent gases. In the case 

 of the alkalies and the alkaline earths, the value of the 

 ionization potential is low and gradually decreases as we 

 proceed to elements with higher atomic weight in the same 

 group, and it is found that the difficulty with which the 

 spectrum can be excited lessens in a parallel manner. Thus 

 under all conditions, if we classify the alkaline earths 

 according to the ease with which they can be excited, the 

 order is Ba, Sr, Ca, and Mg. The same can be said of the 

 other elements. 



In the present paper I have used the word " Stimulus " 

 to denote, in a general manner, all physical agencies tending 

 to make the atoms luminescent. We shall discuss how a 

 high temperature alone can bring about this state. The 

 question can best be approached from the theoretical side. 



Modern spectroscopic works have shown that the lines of 

 an element may be grouped under the following headings*: — 



(1) Lines due to the normal atom. — In this case the lines 

 are produced by the quantum-changes of orbit of the outer- 

 most electron, the nucleus and the remaining electrons 

 behaving as a single charge. The characteristic Rydberg 



number in the series-formula is N( = w — j% — ) • These lines 



are produced under a comparatively low stimulus. 



(2) Lines due to the atom which has lost one electron (the 

 outermost one). — In this case the lines are produced by the 

 quantum changes of orbit of the now outermost electron, 

 the nucleus and the remaining charges behaving as a net 



. * For example, compare Fowler's work on the "Emission Spool nun 

 of Magnesium," Phil. Trans, vol. 214. 



