HI.] THE PRESENT POSITION. 31 



obtained with a lower degree of electrical energy. The spectrum is a 

 mixed one. Even when we take the precaution of throwing an image 

 of the spark on the slit of the spectroscope, the outer cooler layers 

 pierced by the line of sight add their lines to the spectrum of the 

 centre. 



Not only so, but the individuality of the various chemical elements- 

 conies out in a remarkable manner. 



To take one or two instances. I will begin with the gases with a 

 weak and strong induced current. Hydrogen gives us what is termed 

 a structure spectrum, a spectrum full of lines ; this changes to a series.. 

 Oxygen gives us series which change into a complicated line spectrum in 

 which no series has been traced. Nitrogen gives us a fluted spectrum, 

 which changes into a complicated line-spectrum. 



I next pass to the metals, and again, for brevity's sake, I will deal 

 with three substances only. In the case of magnesium, iron, and 

 calcium, the changes observed on passing from the temperature of the 

 arc to that of the spark have been minutely observed. In each new 

 lines are added, or old ones are intensified at the higher temperature. 

 Such lines have been termed enhanced lines. 



These enhanced lines are not seen alone ; as in the case of the spark,, 

 so in the arc outside the region of high temperature in which they are 

 produced, the cooling vapours give us the lines visible at a lower tem- 

 perature. 



Bearing in mind what happens in the case of the gases, we can con- 

 ceive the enhanced lines to be seen alone at the highest temperature in 

 a space sufficiently shielded from the action of all lower temperatures,, 

 but such a shielding is beyond our laboratory expedients ; still, as I 

 shall show, in the atmospheres of the stars we have probably the closest 

 approximation open to our observation of that equally heated space 

 condition to which I have referred. 



The enhanced lines are very few in number as compared with those 

 seen at the temperature of the arc. In the case of iron thousands are 

 reduced to tens. 



The above statements are only general; if we include the non- 

 metals, more stages of temperature are required, and it then becomes- 

 evident that different kinds of spectra are produced at the same tem- 

 perature in the case of different elements ; in other words, at many 

 different heat-levels changes occur, always in one direction, but differing 

 widely for different substances at the lower temperatures. At the 

 highest temperatures at the limit there is much greater constancy 

 in the phenomena observed if we disregard the question of series. If 

 considered from the series point of view, there is no constancy at all. 



It is obvious that with all these temperature effects observed in a- 



