200 



Profs. Liveing and Dewar. 



[May 12 



flirtings in his figure corresponds very well with that we have 

 described in the yellowish-green. The next set, in the green, in his 

 figure does not, however, correspond exactly with ours ; it begins 

 nearer to b than we have observed and consists of four flutings, 

 whereas we observe but two in this set. It looks as if, in his figure, 

 the magnesium-hydrogen spectrum were superposed upon the hydro- 

 carbon spectrum in this region. Further, he gives a third more 

 refrangible set of flutings which we have only observed as two blue 

 bands, not fluted. This third set of flutings, as drawn in his figure, 

 appears to be somewhat more refrangible than the set due to 

 the oxide, and occupies partly the place of the blue hydrocarbon 

 series, but a passage in the text, in which he says that the mag- 

 nesium spectrum of the second order might, without measurement, 

 easily be taken to be identical with that of carbon, almost negatives 

 the supposition that this set of flutings is the blue hydrocarbon set 

 and mistaken for a magnesium spectrum of the first order. To what- 

 ever it may be due, we have not seen anything closely resembling it 

 under the circumstances described by him, though our observations 

 on the spark spectrum of magnesium in hydrogen have now been 

 repeated with all the variations of circumstance which we could 

 devise. 



Mr. Lockyer states (loc. cit.) that none of the lines of magnesium, 

 special to the flame spectrum, are visible in the spectrum of the 

 chloride, either when a flame or a spark is employed. But we find that 

 when the spark is taken between platinum points from a solution of 

 the chloride of magnesium, in a tube such as those used by Delachanal 

 and Mermet, the line at wave-length 5210 can frequently be seen in it 

 when the tube is filled with air, and that if the tube be filled with 

 hydrogen the green flutings of magnesium-hydrogen are persistent 

 and strong. 



Repeated observations have confirmed our previous statements as 

 to the facility with which the magnesium-hydrogen spectrum can be 

 produced in the arc by the help of a current of the gas. In a 

 magnesia crucible, by regulating the current of hydrogen, the flutings 

 can be easily obtained either bright or reversed. 



Comparison of the Spectra. 



When we compare the spectra of magnesium in the flame, arc, and 

 spark, we observe that the most persistent line is that at wave-length 

 2850, which is also the strongest in the flame and arc, and one of the 

 strongest in the spark. The intensity of the radiation of magnesium 

 at this wave-length is witnessed by the fact that this line is always 

 reversed in the flame as well as in the arc when metallic magnesium 

 is introduced into it, and in the spark between magnesium, electrodes 

 when a Leyden jar is used. It is equally remarkable for its power of 



