SIR NORMAN LOCKYER 363 



Gaseous stars Highest temperature. 



Metallic stars , Medium temperature. 



Carbon stars 1 Lowest temperature. 



Hence the differences in- apparent chemical constitutions are as- 

 sociated with differences of temperature. 



Can we associate with the two to which I have already called at- 

 tention still a third class of facts? Laboratory work enables us to 

 do this. When I began my inquiries the idea was, one gas or vapor, 

 one spectrum. We now know that this is not true ; the systems of 

 bright lines given out by radiating substances change with the tem- 

 perature. 



We can get the spectrum of a well known compound substance — 

 say carbonic oxide ; it is one special to the compound ; we increase the 

 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 pur- 

 poses of illustration three substances, and deal with one increase of 

 temperature only, a considerable one and obtainable by rendering a 

 substance incandescent, first by a direct current of electricity, as hap- 

 pens 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. 



