The greater part of this investigation of the spectrum of scandium under different experimental conditions has been based on purified scandia, generously placed at the authors disposal by Sir William Crookes. The principal results are as follows :-1. The arc spectrum of scandium consists of two distinct sets of lines" which behave very differently in solar spectra. Each set includes both strong, and faint lines. 2. Lines belonging to one set correspond with the enhanced lines of other elements,

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... notwithstanding that they appear strongly in the ordinary arc spectrum-* (а) These lines are very feeble or missing from the arc-flame spectrum, and are strengthened in passing to the arc, the arc in hydrogen, or the spark. (б) They occur as relatively strong lines in the Fraunhofer spectrum. (c) They are weakened in the sun-spot spectrum. (d) They occur as high-level lines in the chromosphere. 3. The remaining lines show a great contrast when compared with the first group-(a) They are relatively strong lines in the arc-flame. (&) They are very feebly represented in the Fraunhofer spectrum. (c) The stronger lines are prominent in the sun-spot spectrum. (d) They have not been recorded in the spectrum of the chromosphere. 4. The special development of the enhanced lines in the Fraunhofer spectrum, together with their presence in the upper chromosphere, indicates that the greater part of the scandium absorption in the solar spectrum originates at a higher level than that at which the greater part of the iron absorption is produced. 5. The discussion of scandium lines indicates that while in the case of some elements solar identifications are to be based chiefly on arc lines, in z 2