16 THE ROYAL SOCIETY OF CANADA 



In photographing spectra in the extreme ultraviolet region one 

 is compelled to work with vacuum grating spectrographs, and in 

 working with these instruments, even with the best type of design, 

 one is forced to make two, and possibly three, of the joints gas tight 

 by the use of some kind of wax or other substance which contains 

 hydrocarbons as ingredients. For this reason it is practically im- 

 possible to make certain in taking photographs of the spectra of 

 helium and hydrogen that these gases are absolutely free from carbon 

 in some form or other. It is, therefore not improbable that the view 

 expressed by Millikan is the correct one. 



On the other hand Rutherford"*, in the Bakerian Lecture before 

 the Royal Society of London for 1920, has described a series of experi- 

 ments on the disintegration of atomic nuclei, the results of which 

 have led him to express the opinion that the nuclei of the atoms of 

 carbon are probably made up of two electrons and four subsidiary 

 nuclei of triprotonic helium. If this view should turn out to be 

 correct it is just possible that in the experiments of Millikan on the 

 spectrum of carbon the atoms of carbon, with the powerful "hot" 

 sparks which he used, were disrupted ,into their constituent tri- 

 protonic helium nuclei, and that the wave-lengths which he observed 

 were due to helium in this form. On this hypothesis it would follow 

 that the wave-lengths in question originated in reality in helium 

 atoms and not in those of carbon. This view Millikan considers 

 untenable since he did not find any trace of helium lines in the spectrum 

 of the "hot sparks" in the region between X = 2100 A.U. and X = 700Ô 

 Â.U. 



In view of the uncertainty which, it will be seen from the above, 

 prevails as to the origin of certain wave-lengths obtained in photo- 

 graphing the spectra of helium and carbon, and possibly, too, of 

 hydrogen, an investigation of the spectra of these elements was under- 

 taken by the writers, and in doing so an attempt was made in carrying 

 out the observations with each element to work with the element in 

 as pure a state as possible and as free from contamination with 

 impurities as it was possible to obtain it. An account of this investi- 

 gation follows. 



II. Experiments 



The vacuum grating^ used in this investigation has already been 

 fully described elsewhere. It will sufifice to state here that the 

 grating employed had a radius of 1 metre and a ruling 5.4 cms. wide 



^ Rutherford, Proc. Roy. Soc, Vol. 97, p. 374, 1920. 

 6 McLennan, Proc. Roy. Soc, Vol. 98, p. 114, 1920. 



