•806 Mr. H. Smith : Comparison of Positive Rays 



present; this in spite of the fact that 20 per cent of the 

 mixture is oxygen. 



It is not always a simple matter to say which of two 

 spectra is the stronger. In the case of two spectra yielding- 

 lines evenly dispersed throughout the same regions of wave- 

 lengths in each case, it is of course easy to decide which 

 spectrum is the stronger. In the case, however, where one 

 spectrum contains a few bright lines, and the other a large 

 number of faint lines, any decision must be arbitrary. It is 

 possible, for instance, by varying the conditions of excitation, 

 to vary the relative intensities of the primary and secondary 

 spectra of hydrogen. But at what stage can it be said 

 that the two spectra are equal in intensity? Experimenters 

 confine themselves generally to making observations on the 

 more prominent lines of the spectra dealt with; and in most 

 cases where the masking effect of one spectrum upon the 

 other is large this method is satisfactory, and has led to 

 some important generalizations. 



Previous Work. 



It has been known for some time that when metallic 

 vapours, such as those of cadmium and mercury, are present 

 in the Geissler tube as impurities, the spectra of these metals 

 may be produced in strength, while the spectrum of the gas 

 under examination maybe considerably diminished. Indeed, 

 Prof. Lewis has shown that the presence of one molecule of 

 mercury for every three thousand of hydrogen will cut down 

 -the spectrum of the latter by one half. It has been shown 

 by Nutting that non-metallic elements such as sulphur, 

 bromine, iodine, &c, had similar effects. Even the spectrum 

 of a metallic vapour, like that of sodium, was reduced by the 

 presence of a halogen. Nutting examined altogether fifteen 

 different elements in eighty different combinations; and the 

 conclusion at which he arrived was that if there were an 

 appreciable difference between the atomic weights, then the 

 spectrum of the heavier element masked the spectrum of the 

 lighter. Moreover, the masking effect was roughly pro- 

 portional to the atomic weight. Nutting also observed that 

 on increasing the current density, the spectrum of the lighter 

 element underwent a slightly greater relative increase. It 

 would appear therefore that the masking effect is more 

 noticeable at the smaller current densities. 



It will be noticed that Nutting's result is also confirmed by 

 the case of the electric discharge in air. Under the ordinary 

 induction-coil discharge at low gaseous pressure, the only 



