492 Chronicles of Science. [July, 



some chemists, but branded as compounds by others. In most cases 

 not sufficient was known about them to enable the most impartial 

 judge to decide one way or another. Amongst these two bodies, 

 Erbium and Terbium, have long been kept in the same undecided state, 

 and there is no knowing how many years more the verdict " not 

 proven " might have been recorded against them had not spectrum- 

 analysis come to their aid and substantiated their right to enter the 

 family of elements. M. Bahr first discovered the absorption spectra 

 of Erbium and Terbium, and they have since been submitted to a 

 more extended investigation by M. Marc De la Fontaine. These 

 bodies appear to be intimately associated with the element Didymium, 

 the spectrum of which was discovered by Dr. Gladstone. A very 

 dilute solution of nitrate of didymium shows three rather indistinct 

 black rays, shown in Fig. 3 by the letters a, b, c. In strong solution 

 other lines appear, and the complete spectrum is shown in Fig. 3. On 

 examining a strong solution, a salt of Erbium gives a spectrum having 

 eight bands of absorption, as shown in Fig. 1, and five when in weaker 

 solution. Terbium when in solution gives only the rays shown in 

 Fig 2, and no more make their appearance, however concentrated the 

 liquid is made. 



The positions in respect to the coloured rays of the spectrum 

 may be obtained by reference to the graduated scale, on which the red 

 Lithium line = 10, the yellow Sodium line = 27, and the green 

 Thallium line = 43. 



As will be seen, two bands are common to the three elements, viz. 

 Di b , Tr b , Er a , and Di c , Tr c , Er c , whilst Er f is identical with Di h . 



In a former note we mentioned that Father Secchi had investigated 

 the so-called telluric rays in the solar spectrum, and had arrived at the 

 conclusion that they were due to the absorptive action of aqueous 

 vapour. From a recent communication to the French Academy of 

 Sciences we learn that the Reverend Father has repeated his observa- 

 tions with a more perfect spectroscope, and has been enabled to confirm 

 his former results. 



Dr. Gladstone, F.R.S., in conjunction with the Eev. T. P. Dale, 

 M.A., has been for some time past investigating the Specific Refrac- 

 tive Energies of the Elements and their Compounds. 



In 1863 they communicated a paper on this subject to the Royal 

 Society and at the meeting of the Chemical Society, in May last, some 

 fiu"ther results of their joint labours were brought forward. The 

 " specific refractive energy " of a body is a constant, and not affected 

 by temperature, and is arrived at by dividing the refractive index of 

 the substance (//) minus 1, by the density. The formula already 

 proposed was found to hold good on a more extended investigation of 

 the subject, and the authors generally worked with the fixed line A. 

 The proposition resolved itself into a study of the inquiry whether the 

 specific refractive energy of an element was invariable under all cir- 

 cumstances of isolation or combination, and whether this property in 

 the case of a compound was correctly expressed by taking the mean of 

 the refractive energies of its several elementary constituents. As a 



