Analysis by Spectrum-observations. 505 



V. Reactions of the Rubidium and Ccesium Compounds. 



Caesium and rubidium are not precipitated either by sulphu- 

 retted hydrogen or by carbonate of ammonium. Hence both 

 metals must be placed in the group containing magnesium, 

 lithium, potassium, and sodium. They are distinguished from 

 magnesium, lithium, and sodium by their reaction with bichlo- 

 ride of platinum, which precipitates them like potassium. Neither 

 rubidium nor caesium can be distinguished from potassium by 

 any of the usual reagents. All three substances are precipitated 

 by tartaric acid as white crystalline powders ; by hydrofiuosilicic 

 acid as transparent opalescent jellies, and by perchloric acid as 

 granular crystals; all three, when not combined with a fixed 

 acid, are easily volatilized on the platinum wire, and they all 

 three tinge the flame violet. The violet colour appears indeed 

 of a bluer tint in the case of potassium, whilst the flame of rubi- 

 dium is of a redder shade, and that of caesium still more red. 

 These slight differences can, however, only be perceived when 

 the three flames are observed side by side, and when the salts 

 undergoing volatilization are perfectly pure. In their reactions, 

 then, with the common chemical tests, these new elements can- 

 not be distinguished from potassium. The only method by 

 means of which they can be recognized when they occur together 

 is that of spectrum-analysis. 



The spectra of rubidium and caesium are highly characteristic, 

 and are remarkable for their great beauty. In examining and 

 measuring these spectra we have employed an improved form of 

 apparatus, which in every respect is much to be preferred to 

 that described in our first memoir. In addition to the advan- 

 tages of being more manageable and producing more distinct 

 and clearer images, it is so arranged that the spectra of two 

 sources of light can be examined at the same time, and thus, 

 with the greatest degree of precision, compared, both with one 

 another and with the numbers on a divided scale. 



The apparatus is represented by fig. 12, Plate VI. On the 

 upper end of the cast-iron foot F a brass plate is screwed, 

 carrying the flint-glass prism P, having a refracting angle of 60°. 

 The tube A is also fastened to the brass plate ; in the end of this 

 tube, nearest the prism, is placed a lens, whilst the other end is 

 closed by a plate in which a vertical slit has been made. Two 

 arms are also fitted on to the cast-iron foot, so that they are 

 moveable in a horizontal plane about the axis of the foot. One 

 of these arms carries the telescope J3, having a magnifying 

 power of 8, whilst the other carries the tube C ; a lens is 

 placed in this tube at the end nearest to the prism, and at the 

 other end is a scale which can be seen through the telescope by 



Phil. Mag. S. 4. No. 150. Suppl. Vol. 22. 2 L 



