1881.] on the Origin and Idcniifi/ of Spectra. G89 



tempera til 1*0 of the sj)ark in liydronreii. From Dc La Kiie and Miiller's 

 ol)servations it would appear that nitrogen at a 2)ressiirc of 400 

 millims. should produce much the same elFect on the spark as hydrogen 

 ut 7G0 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 

 temi)erature 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 Ley den jar is used, and the gas is at the atmospheric pressure, 

 the line almost disappears from the spark, to reaj^pear 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-lengtli 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 thus far 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 sufficiently 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 

 production 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 sj^ectra 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 are 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. 



Sparh Spectrum of Magnesium in Eydror/en under increased 

 Pressures. 



In order to ascertain if this peculiar spectrum could be produced 

 at a high temperature in the presence of hydrogen, which we have 

 already shown to be essential to its production at the atmospheric 

 and at reduced pressures, experiments were made with hydrogen at 

 pressures increasing up to twenty atmospheres. 



