Absorption- Spectrum of Cobalt Glass produced by Heat. 319 



diminishes. The band in the red moves towards the least 

 refrangible end of the spectrum, whilst those in the yellow 

 and green retain their position, but become less distinct. 

 Fig. 2 represents the spectrum of the hot glass. 



Fiff. 1. 



Fur. 2. 



As the glass is heated the intensity of its colour decreases 

 as it cools it recovers its original colour, and the absorption 

 spectrum changes back into that represented by fig. 1. 

 The position of the bands 

 reduced to a 

 Cold Glass. 



gives the results, 



Band 



Band 



■•! 



Band III. [ 



was measured, and the table 

 scale of wave-lengths. 



Hot Glass. Dr. Russell's Values. 

 700 712 665 



636 655 632 



608 608 605 



580 583 b88 



565 558 555 



indistinct indistinct 



Dr. "YV. J. Russell, in a paper on the " Absorption Spectra 

 of Cobalt Salts " (Proc. Roy. Soc. xxxii. p. 258) gives a map 

 of the spectrum of cobalt glass drawn to a scale of wave- 

 lengths. The position of the bands, as shown in Dr. Russell's 

 map, is given in the third column of the above table. 



The agreement between the values obtained with the cobalt 

 glass when cold and those from Dr. Russell's map, except 

 for the least refrangible edge of the first band, are as close as, 

 perhaps, could well be expected, considering the small dis- 

 persion of the spectroscope used for the measurements, and 

 the difficulty of determining the exact position of an absorp- 

 tion band. There is considerable difference with regard 

 to the position of the least refrangible edge of band 1 ; that 



