246 Intelligence and Miscellaneous Articles. 



less, and the curve which represents it continues to approach the 

 axis of the temperatures in the part which corresponds to the state 

 of vapour. 



In fine, the rotatory power of substances which I have studied for 

 a definite ray of the spectrum is not a constant ; it varies regularly 

 with the temperature, and changes neither direction, nor virtually 

 intensity, when the liquid passes into the state of vapour. 



For rays of different colours, the law of the deviation of the planes 

 of polarization is independent of the temperature and of the condi- 

 tion of the substance. 



If, then, it be admitted that the rotatory power of active sub- 

 stances depends on their molecular structure, it may be concluded 

 from the preceding that the liquid molecules vaporize without any 

 modification taking place in their form. — Comptes Rendus, June 13, 

 1864. 



INVERSION OF THE ABSORPTION BANDS IN THE SPECTRUM OP 

 DIDYMIUM. BY R. BUNSEN. 



By an experiment which I have made at the instance of Professor 

 Kirchhoff, I have succeeded in changing the dark lines which the 

 absorption spectrum of solutions of oxide of didymium shows into 

 bright lines. If a small quantity of oxide of didymium is heated 

 with microcosmic salt at a red heat until the mass is free from gas- 

 bubbles and has become transparent, an amethyst-coloured glass is 

 obtained on cooling, which, held between the slit of the spectrum- 

 apparatus and the source of light, produces the characteristic ab- 

 sorption spectrum of didymium compounds. As source of light, an 

 ignited platinum wire as fine as a hair is used, the image of which, 

 by means of a small lens of short focal distance, is made to fall upon 

 the slit in a suitable manner. If then the didymium glass melted in 

 a platinum spiral is brought between the source of light and the lens, 

 the stronger absorption lines of didymium, and more especially the 

 chief band aDi near Fraunhofer's line D, are seen in the spectrum- 

 apparatus with perfect distinctness. If the didymium glass in the 

 spiral (which issustained by a holder) is gradually heated by a non- 

 luminous flame held below it, so long as a red heat has not been 

 attained the band aDi is seen to become gradually broader. If the 

 temperature is raised to a continually increasing red heat, the dark 

 band diminishes more and more in darkness, and finally entirely dis- 

 appears. If now the platinum wire which serves as hindermost 

 source of light is removed, a spectrum of the fused ignited didymium 

 glass appears, which has exactly, in the position of the dark band 

 aDi in the absorption spectrum, a similarly shaped bright line on a 

 dark ground. Indications of a similar inversion may also be perceived 

 in the other absorption lines of the same spectrum. 



Professor Bahr in Upsala made two years ago the interesting 

 observation that the salts of the metals occurring along with yttrium, 

 and designated by Mosander erbium and terbium, give remarkable 

 absorption spectra which are wanting in solutions of pure yttria. 



