170 THE CHEMISTRY OP THE STARS. 



temperature so as to break up the compound, and we then get the 

 spectra of its constituents, carbon and oxygen. 



But the important thing in the present connection is that the spectra 

 of the chemical elements behave exactly in the same way as the spectra 

 of known compounds do when we employ temperatures far higher than 

 those which break up the compounds; and indeed in some cases the 

 changes are more marked. For brevity I will take for purposes of 

 illustration three substances, aud deal with one increase of temperature 

 only, a considerable one and obtainable by rendering a substance incan- 

 descent, first by a direct current of electricity, as happens in the 

 so-called "arc lamps" employed in electric lighting, and next by the 

 employment of a powerful induction coil and battery of leyden jars. 

 In laboratory parlance we pass thus from the arc to the jar-spark. 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; outside the region of high 

 temperature in which they are produced, the cooling vapors give us the 

 cool lines. Still we can conceive 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. 



In watching the appearance of these special enhanced lines in stellar 

 spectra we have a third series of phenomena available, and we find that 

 the results are absolutely in harmony with what has gone before. Thus : 



Gaseous stars.. Highest temperature.. Strong helium and faint enhanced lines. 



n/r„+„n- i. i\^ j- j- a S Feeble helium and strong enhanced lines. 



Metallic stars.. Medium temperature.. < XT ■, -,- -, , *=•.. 



1 } No helium and strong arc lines. 



Carbon stars.. Lowest temperature. .. Faint arc lines. 



It is clear now, not only that the spectral changes in stars are associ- 

 ated with, or produced by, changes of temperature, but that the study 

 of the enhanced spark and the arc lines lands us in the possibility of a 

 rigorous stellar thermometry, such lines being more easy to observe 

 than the relative lengths of spectrum. 



Accepting this, we can take a long stride forward and, by carefully 

 studying the chemical revelations of the spectrum, classify the stars 

 along a line of tern perature. But which line? Were all the stars in popu- 

 lar phraseology created hot? If so, we should simply deal with the run- 

 ning down of temperature, and because all the hottest stars are chem- 

 ically alike, all cooler stars would be alike. But there are two very 

 distinct groups of coolest, stars ; and since there are two different kinds 

 of coolest stars, and only one kind of hottest stars, it can not be merely 

 a question either of a running up or a running down of temperature. 



