1880.] On the Spectra of Magnesium and Lithium. 



97 



ture of tlie spark in hydrogen. From De La Rue and Miiller's obser- 

 vations it would appear that nitrogen at a pressure of 400 millims. 

 should produce much the same effect on the spark as hydrogen at 

 760 millims. Now the pressures of the nitrogen and carbonic oxide 

 were reduced far below this without any trace of the line in question 

 being visible. Moreover, the magnesium line at 4481, which is not 

 seen in the arc, and may be reasonably ascribed to the higher tem- 

 perature of the spark, may be seen in the spark at the same time 

 as the line at 5210 when hydrogen is present. Nevertheless tem- 

 perature does seem to affect the result in some degree, for when a 

 large Leyden jar is used, and the gas is at the atmospheric pressure, 

 the line almost disappears from the spark, to re-appear when the 

 pressure is reduced ; but by no variation of temperature have we 

 been able to see the line when hydrogen was carefully excluded. 



A line of the same wave-length has been seen by Young in the 

 chromosphere once. Its absence from the Fraunhofer lines leads to 

 the inference that the temperature of the sun is too high (unless at 

 special times and places) for its production. If it be not due to a 

 compound of magnesium with hydrogen, at any rate it occurs with 

 special facility in the presence of hydrogen, and ought to occur in the 

 sun if the temperature were not too high. 



We have been careful to ascribe this line and its attendant series 

 to a mixture of magnesium and hydrogen rather than to a chemical 

 compound, because this expresses the facts, and we have not yet 

 obtained any independent evidence of the existence of any chemical 

 compound of those elements. We have independent evidence that 

 mixtures which are not probably chemical compounds favour the pro- 

 duction of certain vibrations which are not so strong or are not seen 

 at all when the elements of those mixtures are taken separately. The 

 remarkable absorptions produced by mixtures of magnesium with 

 potassium and sodium above-mentioned belong to this class. We have 

 not been able to obtain the emission spectra corresponding to these 

 absorptions, but in the course of our observations on the arc we have 

 frequently noticed that certain lines of metals present in the crucible 

 are only seen or come out with especial brilliance when some other 

 metal is introduced. This is the case with some groups of calcium 

 lines which are not seen, or barely visible, in the arc in a lime 

 crucible, and come out with great brilliance on the introduction of a 

 fragment of iron, but are not developed by other metals such as tin. 



The spectrum of magnesium, as seen in the arc, contains several 

 lines besides those heretofore described. These lines come out 

 brightly, generally considerably expanded, when a fragment of mag- 

 nesium is dropped into the crucible through which the arc is passing, 

 but rapidly contract and gradually become very faint or disappear 

 entirely. 



vol. xxx. H 



