ly^l-] on the Orijin and Identity of Spectra. C97 



Almost equal iu persisteuce arc the series of triplets. Only the 

 least refrangible pair of these triplets are seen iu the flame, another 

 pair are seen in the spark, but the eoni])lete series is only seen in the 

 arc. We regard the triplets as a series of harmonics, and to account 

 for the whole series being seen only in the arc we must look to some 

 other cause than the temperature. This will probably bo found in 

 the greater mass of the incandescent matter contained in the crucible 

 in which the arc was observed. 



The blue line of the flame at wave-length 4570 is. well seen in the 

 arc, and is easily reversed, but is always a sharp line, increased in 

 brightness but not sensibly expanded by putting magnesium into the 

 crucible. In the spark, at atmospheric pressure, it is only seen close 

 to the pole or crossing the field in occasional flashes; but seems 

 to come out more decidedly at rather higher pressures, at least in 

 hydrogen. 



The series of bands near L, well developed in the flame, but not 

 seen at all in the arc or sjiark, look very much like the spectrum of a 

 compound, but we have not been able to trace them to any particular 

 combination. Sparks in air, nitrogen, and hydrogen have alike 

 failed to produce them. The very strong, rather diftuse triplet at M, 

 with which they end, so closely resembles in general character the 

 other magnesium triplets, that it may well be connected with that 

 constitution of the magnesian particle which gives rise to the triple 

 sets of vibrations in other cases, but, if so, its presence in the flame 

 alone is not easily explained. 



The occurrence of this triplet in the ultra-violet, and of the 

 remarkable series of bands associated with it, as well as the extra- 

 ordinary intensity of the still more refrangible line at wave-length 

 2850, which is strongly reversed in the spectrum of the flame, 

 corroborates what the discovery of the ultra-violet spectrum of water 

 had revealed, that at the temperature of flame substances while 

 giving in the less refrangible part of the spectrum more or less 

 continuous radiation, may still give, in the regions of shorter wave- 

 length, highly discontinuous spectra, such as have formerly been 

 deemed characteristic of the highest temperatures. This subject we 

 will not discuss further at present, but simply remark that " it opens 

 up questions as to the emissive power for radiation of short wave- 

 lengths of gaseous bodies at the comparatively low temperature of 

 flame with regard to which we are accumulating facts." 



In the arc and spark, but not in the flame, we have next a very 

 striking group of two very strong lines at wave-lengths about 2801 

 and 2794, and a quintuple group of strong but sharp lines above 

 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. Two blue lines in the 

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



