198 Mr. H. C. Sorby on some Spectra of Compounds [Feb. 10, 



analogous to what occurs in the case of solid and powdered crystals of 

 sulphate of didymium ; for the absorption-bands in the spectrum of the 

 light transmitted hy a thin layer of the fine powder, strongly illuminated 

 from the other side, are as distinct as in that transmitted by a many times 

 greater thickness of solid and transparent crystal. We may very conveniently 

 take advantage of this fact in studying the spectra of such substances, when 

 the amount of material at command is otherwise too small. This seems to 

 be because the transmitted light does not simply pass through the crystals, 

 but is in great measure reflected from them backwards and forwards, and 

 thus, as it were, passes through a greater thickness. It is also to a con- 

 siderable extent similar to that reflected from the powder when illuminated 

 from above, as may be clearly proved by what occurs in the case of uranic 

 salts. These when in a state of moderately fine powder transmit light, 

 giving a spectrum showing not only the absorption-bands in the blue, which 

 alone are met with in that transmitted by a clear crystal, but also the bands 

 in the green, which depend on fluorescence, characteristic of that reflected 

 from the powder*. These two kinds of bands can be easily distinguished 

 by means of a plate of deep blue cobalt glass, which has an entirely different 

 action, according as it is placed below or above the object when the bands 

 are due to fluorescence, but has no such effect when they are due to ordinary 

 absorption. It would perhaps be well to mention here that I have in this 

 manner proved that the abnormal bands seen in the spectra of the com- 

 pounds of zirconia with the oxides of uranium described in this paper are 

 due to genuine absorption, and not to fluorescence. 



The remarkable spectrum of somejargons has been already described by 

 me in the 'Chemical News 't, and in the Proceedings of the Royal Society 

 One of its most striking peculiarities is that when light passes in a direction 

 perpendicular to the principal axis of the crystal, and the spectrum is divided 

 by means of a double-image prism into two spectra, having the light polarized 

 in opposite planes, though some of the absorption-bands are of equal inten- 

 sity in both images, yet others are comparatively absent, some in one and 

 some in the other ; whereas in the case of other dichroic crystals which 

 give spectra with absorption-bands, they are usually all more distinct 

 in one image than when the light is not polarized, and all fainter, or even 

 comparatively absent, in the other. No sooner had I observed this spectrum 

 (No. 5, given below), than I made various experiments in order to ascertain 

 whether uranium was present or not ; and the then known tests that could 

 be applied to the amount of material at my command seemed to show that 

 it was absent. This was quite in accord with the results of the various 

 analyses published by other chemists, none of whom mention the existence 

 of any trace of that substance. Moreover the general character of the 

 spectrum was entirely unlike that of all the known compounds of uranic 

 oxide. The various artificial salts all agree in giving a variable but small 



* See Stokes's papers, Phil. Trans. 1852, p. 463, and 1853, p. 392. 

 f Yol. six. p. 122. % Vol xvii. p. 511. 



