ON OUIl KNOWLEDGE OF SPECTllUM ANALYSIS. 269 



Dewar there is another band still farther in the ultra-violet, and apparently 

 coinciding with the solar line P. 



Spectmm of Carbonic Oxide. — It has been said already that great care 

 must be taken, in order to produce a spectrum of oxygen, to exclude all 

 carbon impurities. If this precaution is not taken a spectrum is obtained 

 which no doubt belongs to carbonic oxide. The spectrum is most bril- 

 liantly obtained in a Pliicker's tube filled with carbonic oxide. The spec- 

 trum was carefully examined by Wiillner, and was measured by Watts as 

 well as Angstrom and Thalen. As some of the bands are situated rather 

 near to the bands of the candle-spectrum, the two spectra have often been 

 confounded, and we therefore give the wave-lengths obtained by the 

 Swedish observers for the most conspicuous bands: 5607-5 ; 5197-0; 

 4833-0. 



Spectrum of Carbonic Acid. — Pliicker mentions already m his first paper 

 on the spectra of gases (1858) that carbonic acid in a vacuum-tube shows 

 a baud in the red which is very strong at first and gradually disappears. 

 This band he attributes to carbonic acid (1859), which, it is known, is 

 gradually decomposed by the spark. Wiillner has carefully examined 

 and described the changes going on in the spectrum seen in a vacuum-tube 

 when it is first filled with carbonic acid. 



Professor Piazzi Smyth (' Phil. Mag.' xlix. p. "24) has given some very 

 careful and valuable measurements of the details in some of the flutings 

 of the spectra which we have described. He ascribes the candle spec- 

 trum to a hydrocarbon, and the spectrum which we have put down as 

 belonging to carbonic oxide, he refers to carbon, as it is also visible in 

 tubes not containing any oxygen. Professor Piazzi Smyth has, however, 

 not filled his own tubes, and we must be careful not to attach too much 

 value to the labels put on vacuum tubes by the glass-blower who has 

 filled them. According to Watts, a tube containing hydrocarbons does 

 not show this spectrum when the gas is heated in contact with metalHc 

 sodium.' 



V. Chlorine. 



V. d. Willigen : ' Pogg. Ann.' cvi. p. 624 (1859). 



Pliicker: 'Pogg. Ann.' cvii. p. 528 (1859). 



Pliicker and Hittorf : 'Phil. Trans.' civ. p. 24 (1865). 



Salet : ' Ann. Chim. Phys.' xxviii. p. 24 (1873). 



Ciamician : ' Wien. Ber.' Ixxviii. (II.) p. 872 (1873). 



Morren: 'C. K' Ixviii. p. 376 (1869). 



Gernez: ' C. R.' Ixxiv. p. 660 (1872). 



W. A. Miller : 'Phil. Mag.' xxvii. p. 81 (1845). 



The Line-spectrum. — This is the spectrum which is obtained if an 

 electric spark is taken in chlorine gas. It has been mapped by Pliicker 

 and Hittorf, whose measurements have been reduced to wave-lengths by 

 Watts. Some earlier measurements of the strongest lines will be found 

 in Pliicker's paper. Salet has also mapped this chlorine spectrum as well 

 as could be done with a spectroscope of small dispersive powers. A few 

 of the lines have been measured by Angstrom (' Phil. Mag.' xlii. p. 398), 

 None of these measurements lay claim to great accuracy. Recently 

 Ciamician has given a detailed account of the successive changes which 

 this spectrum undergoes, if the pressure is either greatly reduced or 

 ' PMl. Mag. xlviii. p. 45G 1874). 



