Chemical Constitution of Saline Solutions. 289 



Band V. (Blue). 



At 20°, . . . . 488 to 486, extending to 481 

 At 100°, . . . . 488 to 486, „ 481 



There is a praseodymium line at 4823, and Thal^n gives an erbium line at 

 487-7 to 486-5. 



Band VI. (Blue). 



At 20° 478 to 477 



At 100°, . . . 477 to 474 



Probably praseodymium, 4823 to 4759. 



Band VII. (Blue). 



At 20°, . . . . 467 to 455 

 At 100°, . . . . 469 to 453 



There is a praseodymium band hereabouts, 4692. 



Band VIII. (Indigo). 



At 20°, . . . . 403 to 449 (Neodymium). 

 At 100°, . . . .All absorbed. Notbiug visible. 



As already mentioned in the Introduction, Bunsen,* in 1886, examined the 

 absorption spectra of salts of didymium both in the state of crystals and in 

 solution, and found that they presented several differences. The width of the 

 absorption bands varies with the thickness of the layer of solution and the quantity 

 of the salt contained in it. Solutions of the chloride sulphate and acetate, each 

 containing the same quantity of didymium, yielded different spectra, the bands 

 being shifted towards the red with increase in the molecular weight of the salt. 



It may be mentioned that H. Becquerelf more recently has observed such 

 variations both in the crystallized salts and in their solutions. In any one salt 

 the positions of the bands differ from those of the same salt in solution, and 

 both in the solid and the state of solution the different salts slightly vary. In 

 the measurements which he made he distinguishes the praseodymium from the 

 neodymium, though the two were not separated. 



But Becquerel's measurements were made with too small a dispersion to show 

 any marked differences in the spectra of the solutions ; and such differences as 

 occur in the solids were shown by Bunsen to be due to double refraction, the 

 ordinary and extraordinary rays exhibiting spectra with the same bands, but. in 

 different positions. 



* Ueber die Erscheinungen beim Absorptions-Spectrum des Didyms. Pogg. Ann., vol. 128, p. 100. 

 f " Sur les variations des spectres d' absorption du didyme." Comptes Kendus, vol. 144, pp. 777-780 and 

 1691-1693. 



