270 BEPOKT— 1880. 



increased. Lines, Avhich are visible at one pressure, altogether disappear 

 at another. Some preliminary experiments have convinced the writer of 

 this report, that we have here to deal with a mixture of several overlap- 

 ping spectra. In reality the phenomena are even more complicated than 

 Ciamician supposes, but a more extensive series of experiments is required 

 before any detailed account can be given. 



The Band-spectrum. — This is the spectrum Avhich is obtained by ab- 

 sorption, if sunlight is sent through a long column of chlorine gas. The 

 spectrum was first observed by Morren, who describes it, but does not give 

 any measurements. It has never been obtained as an emission spectrum. 



Compounds of CJdorine and Oxijgen. — The absorption spectra of chlorine 

 trioxide and chlorine peroxide were examined by Prof. "W. A. Miller in 

 184-5 and found to be identical, while chlorine monoxide did not show 

 any bands. As no other case is known in which two different compounds 

 give the same spectrum, and as the oxides of chlorine are very unstable, 

 there is no doubt that the spectrum of one of them only was observed, 

 that gas to which the spectrum belongs being also present when the other 

 oxide was examined. Gernez confirming Miller's results, also found that 

 a weak solution of these gases in some liquids presents the same absorption 

 bands. According to Gernez a long tube filled with chlorine monoxide 

 shows the same spectrum. A drawing of the spectrum will be found in 

 Miller's paper. 



VI. Br 



amine. 



Pliicker: 'Pogg. Ann.' cvii. p. 527 (18-59). 



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



Salet: 'Ann. Chim. Phys.' xxviii. p. 26 (1873). 



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



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



Eoscoe and Thorpe : 'Phil. Trans.' clxvii. p. 207 (1876). 



Moser : ' Pogg. Ann.' clx. p. 177 (1877). 



Ciamician: ' Wien. Ber.' Ixxviii. (II.) p. 874 (1878). 



Hasselberg: 'Mem. de St. Pet.' xxvi. 4 (1878). 



_ The Line-specirmn. — We only possess approximate measurements of 

 this spectrum by the same authors who mapped the chlorine spectrum. 

 The spectrum appears whenever the electric discharge passes through 

 the vapour of bromine. Ciamician has observed similar changes in tiie 

 spectrum of 'bromine to those already mentioned in chlorine. 



The Band-spectrum. — This spectrum is obtained by absorption. It 

 was first observed by Prof. W. H. Miller in 1833. 



Drawings and measurements have been made by Roscoe and Thorpe 

 and Moser, who mentions some changes which the spectrum shows on 

 being heated. The most detailed and apparently the best drawings ai-e 

 given by Dr. B. Hasselberg. Both Moser and Hasselberg's measurements 

 begin in the orange, so that for absorption-bands in the red we have to 

 refer to Roscoe and Thorpe's map. 



A flame of hydrogen containing bromine gives a continuous spectrum 

 only._ Similarly, if a hard glass tube is heated to a low red heat and 

 bromine introduced, the gas becomes luminous ; but a continuous spectrum 

 only is seen. It is uncertain whether this continuous spectrum is due only 

 to the bands of the absorption spectrum widened by an increase of 

 temperature, or whether we have to deal with a true continuous spectrum. 



