AND ITS RELATION TO SOLAR SPECTRA. 53 



A valuable confirmation of the identification of the enhanced lines was thus obtained, 

 but as regards the remaining lines it can only be said that, within the limits of these 

 experiments, the brighter arc lines survived in the " arc-flame " with little change in 

 their relative intensities. 



The arc-flame spectrum may accordingly be regarded as consisting of the brighter 

 lines of the arc spectrum, except that the enhanced lines (indicated by " p " in the 

 second column of Table I.) are entirely absent. Separate estimates of intensities in 

 the arc -flame* would therefore be superfluous, and might be misleading. The principal 

 lines of the arc-flame spectrum are brought together in Table IV. 



Impurities. 



Allowing for the impurities known to be introduced by the use of carbon poles, or 

 by admixture with other substances for the special purposes already mentioned, there 

 is no evidence of any considerable impurity in the scandia so carefully prepared by 

 Sir WILLIAM CROOKES. In the blue end of the spectrum, thanks chiefly to the 

 admirable work of EXNER and HASCHEK, fairly complete data are available for the 

 detection of such impurities, and in this region at least, with the exception of the 

 yttrium line 4883'8, all the probable impurity lines are extremely faint. In the less 

 refrangible parts of the spectrum the existing records do not permit the identification 

 of impurities to the same extent, but from the evidence afforded by the blue end it is 

 unlikely that any but very faint lines will turn out to be due to substances other 

 than scandium. 



A list of the lines rejected as impurities (excluding those of calcium, barium, iron, 

 sodium, and potassium introduced in the course of the observations) is given in 

 Table II. It will be seen that the principal impurity lines are attributed to yttrium, 

 ytterbium, thorium, and cerium, while a few lines are probably due to samarium, 

 gadolinium, and europium. Other faint lines at present included in the general list 

 of scandium lines, Table I., may subsequently have to be rejected as impurity lines, 

 more especially those which occur in the region less refrangible than 4700. 



As will be seen from Table I., not more than two of the lines given by EXNER and 

 HASCHEK do not occur in Sir WILLIAM CROOKES' scandia, while several of LOCKYER 

 and BAXANDALL'S lines have not been found. The scandium oxalate with which the 

 latter observers worked was admittedly impure, and it would appear probable that 

 lines given by them which do not occur in my own list are not due to scandium. As 

 no list of rejected lines was given by LOCKYER and BAXANDALL, it cannot be 

 determined to what extent the additional lines in my table should be attributed to 

 impurities on similar grounds. 



As already remarked, impurity lines in my own catalogue are probably only to be 

 expected among those of very low intensity, but several well-marked lines are not 

 recorded by LOCKYER and BAXANDALL. In reply to an inquiry as to three of these 



