AND ITS RELATION TO SOLAR SPECTRA. 51 



nation of several negatives made with varying exposures, as there is a tendency for 

 small differences in the brighter lines to be effaced by long exposure. It should be 

 understood, however, that the intensities estimated in this way indicate little more 

 than the relative brightnesses of the lines in the same part of the spectrum. 



The enhanced lines, of which 5527 - may be taken as a type, fluctuated consider- 

 ably in intensity in the arc and appeared differently on different photographs. 

 Nevertheless, all the lines of this class varied together, so that there was no change 

 in the intensities of the enhanced lines with respect to each other, but only with 

 respect to the arc lines. 



The flutings which occur in the arc spectrum are dealt with under a separate 



heading. 



The Arc in Hydrogen. 



The well-known experiments of CREW* and othersf have shown that, when the arc 

 is surrounded by an atmosphere of hydrogen, the spectrum changes in the direction 

 of that given by the spark. Flame lines are relatively reduced and enhanced lines 

 increased in intensity. 



Photographs of the scandium arc in hydrogen, at a pressure of about 75 mm., were 

 taken, and the enhanced lines were readily identified by their strengthening when 

 observed in this way. The use of the arc in hydrogen probably does not reveal 

 anything more than the spark as regards the line spectrum, but is sometimes a 

 convenient method of arriving at the same result. Scandia, however, was rather 

 refractory under this treatment, and it was not found possible to avoid the presence 

 of carbon flutings in the photographs. 



An interesting result of this experiment was the complete disappearance of the 

 scandium flutings which form such a striking feature of the arc in air. 



Although the enhanced lines as a whole were brightened when the. arc was passed 

 in hydrogen, their relative intensities were not appreciably different from those found 

 in the ordinary arc. It would therefore serve no useful purpose to give separate 

 estimates of the intensities of the lines in hydrogen. Following LOCKYER'S notation, 

 the enhanced lines are indicated, in the second column of Table L, by the letter " p" 

 following the intensity number, p being an abbreviation for " proto," so that pSc 

 signifies " protoscandiurn," and so on. 



For the more refrangible part of the spectrum (3930 to 4670) the enhanced lines 

 were identified first by their behaviour in the arc-flame, as explained under the next 

 heading, and checked by reference to a photograph of the spark spectrum which was 

 kindly forwarded to me by Sir WILLIAM CROOKES. 



It should be noted that the enhanced lines of scandium differ from those of iron, 

 titanium, and certain other elements, in exhibiting themselves with relatively great 



* ' Astrophys. Jour.,' vol. 12, p. 167 (1900). 

 t ' Roy. Soc. Proc.,' vol. 72, p. 253 (1903). 

 H 2 



