202 Investigations on the Spectrum of Magnesium. [May 12, 



them. The former are usually reversed in the spark with jar, and all 

 are reversed in the arc when much magnesium is present. There are 

 also several single lines in the visible part of the spectrum common to 

 the arc and spark. All of these may be lines developed by the high 

 temperature of the arc and spark. An indigo and a violet line in the 

 arc have not been traced in the spark, but their non-appearance may 

 be due to the same cause as that above suggested for the non- 

 appearance of the higher triplets, the smallness of the incandescent 

 mass in the spark. 



A pair of lines in the are near U appear to be represented in the 

 spark by an equally strong, or stronger, pair near but not identical in 

 position. The possibility of such a shift, affecting these two lines 

 only in the whole spectrum and affecting them unequally, must in the 

 present state of our knowledge be very much a matter of speculation. 

 Perhaps sufficient attention has not hitherto been directed to the 

 probability of vibrations being set up directly by the electric discharge 

 independently of the secondary action of elevation of temperature. 

 Some of the observations above described, arid many others well 

 known, indicate a selective action by which an electric discharge 

 lights up certain kinds of matter in its path to the exclusion of others ; 

 and it is possible that in the case of vibrations which are not those 

 most easily assumed by the particles of magnesium, the character of 

 the impulse may slightly affect the period of vibration. The fact that, 

 so far as observations go, the shift in the case of this pair of 

 magnesium lines is definite and constant, militates against the supposi- 

 tion suggested. On the other hand, the ghost-like pairs of lines 

 observed in the spark below the triplet near L, suggest the idea that 

 some of the particles have their tones flattened by some such cause. 



The strong pair at wave-length 2801, 2794, are accompanied in the 

 spark, but not in the arc, by a much feebler, slightly more refrangible 

 pair, but these have not the diffuse ghost-like character of those just 

 alluded to. 



These lines are phenomena of the high potential discharge in which 

 particles are torn off the electrodes with great violence, and may well 

 be thrown into a state of vibration which they will not assume by 

 mere elevation of temperature. 



There are two lines in the spark besides the well-known line at 

 wave-length 4481 which have not been observed in the arc, but they 

 are feeble and would be insignificant if it were not the fact that they, as 

 well as the line at wave-length 4481, all short lines seen generally only 

 about the poles, appear to be present in the solar spectrum. In the sun 

 we seem to have all the lines -common to the flame, arc, and spark (unless 

 the line given in Angstrom's map at 4570*9 be not identical as we believe 

 it to be with the magnesium line), and possibly, judging by Rutherford's 

 photograph, the strong triplet of the flame at M ; but one line common 



