ON OUR KNOWLEDGE OF SPECTRUM ANALYSIS. 271 



In the latter case we should have the remarkable fact of a vapour giving a 

 continuous spectrum at a higher temperature than the one at which it 

 gives the band spectrum. 



VII. Iodine. 



Pliicker: ' Pog-g. Ann.' cvii. p. (338 (1859). 



Wiillner: ' Pogg. Ann.' cxx. p. 158 (1863). 



Mitcherlich: ' Pogg. Ann.' cxxi. p. 474 (1864). 



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



Salet : ' C. R.' ixxiv. p. 1249 ; 'Ann. Chim. Phvs.' xxviii. p. 29 (1872). 



W. H. Miller : ' Phil. Mag.' ii. p. 381 (1833). 



W. A. Miller : ' Phil. Mag.' xxvii. p. 86 (1845). 



Thalen : ' Stockholm Akad. Handl.' viii. (1870). 



Lockyer: ' Proc. Roy. Soc' xxii. p. 377 (1874). 



Lockyer and Roberts : ' Proc. Roy. Soc' xxiii. p. 348 (1875). 



Ciamician : ' Wien. Ber.' Ixxviii. (II.) p. 877 (1878). 



The Line-spectrum. — -This spectrum, which appears under the same 

 circumstances as the line-spectra of chlorine and bromine, has been mapped 

 by the same observers. Ciamician found in this spectrum similar changes 

 as when the pressure was greatly reduced. 



The Band-specfrum. — This spectrum, which is easily obtained as an 

 absorption-spectrum at low temperatures, was first observed by Prof. 

 W. H. Miller, 1833. It has been carefully mapped by Thalen. The 

 darkening and widening of the bands when the temperature is increased 

 has been described by Thalen and Lockyer. It is a curious fact that the 

 absorption on heating extends continuously into the blue and violet (which 

 are clear at low temperature) so that the whole of the mare refrangible 

 end of the spectrum can be blocked out. At higher temperatures, however, 

 the violet and blue light is transmitted, and the spectrum resembles again 

 that of low temperature. Thus Lockyer heating various vapours in an 

 iron tube placed inside a furnace, supplied with coke or chai'coal, found 

 that iodine gave an intense bank of general absorption in the violet, where 

 at the ordinary temperature the vapour transmits light. In his joint 

 experiments with Mr. Chandler Roberts, in which the vapours were heated 

 to a still higher degree he found that the violet and blue light was trans- 

 mitted again. 



Salet could obtain this spectrum as an emission spectrum in the wide 

 part of a Geissler tube. Also by heating the vapour of iodine round a 

 Avhite-hot platinum spiral. 



Wiillner, however, was the first to observe this spectrum as an emission 

 .spectrum by charging a hydrogen flame with iodine vapour. 



Angstrom had already examined the spectrum of an alcohol flame 

 containing iodine, but had obtained a difi'erent spectrum, showing bands 

 in the green. According to Mitcherlich the latter spectrum is obtained if 

 a flame of hydrogen contains small quantities of iodine. If large quantities 

 are present the reversal of the absorption spectrum is seen. Salet did not 

 observe this spectrum ; but the one mentioned by Angstriun and Mitcherlich, 

 which very likely is that of some compound of iodine. 



Spectrum of Iodine Chloride. — The absorption si^ectrum of the vapour 

 of iodine chloride was observed by Gernez (' C. R.' Ixxiv. p. 660) and 

 mapped by Roscoe and Thorpe, who drew attention to the resemblance of 

 this spectrum to that of bromine. ('Phil. Trans.' clxvii. p. 207.) 



