CREW AND BAKER. — SPARK SPECTRUM OP CARBON. 401 



3. Exposure begins ^ second after breaking of arc, and lasts for 1 

 second ; middle of exposure one second after beginning of spark. Here 

 the image of the spark is barely visible just before the slit is covered. 

 The spark is distinctly louder than in the preceding phases. 



4. Middle of exposure l^ seconds after beginning of sparlj;. 

 o. Middle of exposure 1| seconds after beginning of spark. 



6. Middle of exposure 2| seconds after starting spark. 



7. Middle of exposure 5;^ seconds after starting spark. Here the 

 electrodes begin to show merely red, instead of white, hot. 



8. Middle of exposure 7^ seconds after beginning: spark distinctly 

 noisy. 



9. The last photograph in the series was taken at twelve seconds 

 after the beginning of the spark, the duration of the exposure being, as 

 in the preceding cases, one second. Even at this late stage a distinct 

 crescendo is still noticeable in the noise of the spark. 



The enormous increase of brilliancy from the hot spark to the cold 

 may be judged from the fact that in order to make the cyanogen band at 

 X 3883 of uniform intensity the exposure time for the first of the series 

 was eight hours and for the last of the series twenty minutes. 



Results. 



As in the case of the Swan spectrum and the carbon arc, so also in the 

 case of the carbon spark, the flutings are, of course, the dominant fea- 

 tures of the entire spectrum. The first question, therefore, which natu- 

 rally arises, in the development of the spark, is concerning the order and 

 the relative intensity in which these cyanogen bands make their appear- 

 ance. A second question might be asked concerning the stage at which 

 the air lines make their appearance. A third query is when and how do 

 the numerous metallic impurities present themselves? Our photographs 

 permit at least partial answers to these three questions for the region 

 lying between A4o0() and X 3000. The phenomenon is one wiiich 

 cannot be accurately observed by the eye, and the exposure times are 

 so long as to render photographing in the visible region well-nigli 

 impracticable. 



I. Carbon Flutings and Lines. 



The cyanogen bands at XX 4216, 3883, and 3590 all make their 

 appearance on the first photograph of the series. Their relative inten- 

 sity is practically the same as in the case of the spark between cold 

 electrodes, which, for the sake of brevity, we shall hereafter call the 

 VOL. xxxviii. — 2G 



