Analysis by Spectrum- observations. . 509 



these lines most distinctly. Sulphate of rubidium and similar 

 salts also give very beautiful spectra. Even silicate and phosphate 

 of rubidium yield spectra in which all the details are plainly seen. 



The spectrum of caesium is especially characterized by the two 

 blue lines Csa and Cs/3 ; these lines are situated close to the 

 blue strontia line SrS, and are remarkable for their wonderful 

 brilliancy and sharp definition. The line CsS, which cannot be 

 so conveniently used, must also be mentioned. The yellow and 

 green lines represented on the figure, which first appear when 

 the luminosity is great, cannot so well be employed for the pur- 

 pose of detecting small quantities of the csesium compounds ; but 

 they may be made use of with advantage as a test of the purity 

 of the csesium salt under examination. They appear much more 

 distinctly than do the yellow and green lines in the potassium- 

 spectrum, which, for this reason, we have not represented. 



As regards distinctness of the reaction, the csesium com- 

 pounds resemble in every respect the corresponding rubidium 

 salts : the chlorate, phosphate, and silicate gave the lines perfectly 

 clearly. The delicacy of the reaction, however, in the case of 

 the csesium compounds is somewhat greater than in that of the 

 corresponding compounds of rubidium. In a drop of water 

 weighing 4 milligrammes, and containing only O0002 milli- 

 gramme of chloride of rubidium, the lines Rba and Rb/3 can 

 only just be distinguished ; whilst O00005 milligramme of the 

 chloride of csesium can, under similar circumstances, easily be 

 recognized by means of the lines Csa and Csj8. 



If other members of the group of the alkaline metals occur 

 together with csesium and rubidium, the delicacy of the reaction 

 is of course materially impaired, as is seen from the following 

 experiments, in which the mixed chlorides contained in a drop 

 of water weighing about 4 milligrammes, were brought into the 

 flame on a platinum wire. 



When O003 milligramme of chloride of csesium was mixed with 

 from 300 to 400 times its weight of the chlorides of potassium 

 or sodium, it could be easily detected. Chloride of rubidium, on 

 the other hand, could be detected with difficulty when the quan- 

 tity of chloride of potassium or chloride of sodium amounted to 

 100 to 150 times its weight of the chloride of rubidium employed. 



0*001 milligramme of chloride of csesium was easily recog- 

 nized when it was mixed with 1500 times its weight of chloride 

 of lithium ; whilst 0'001 milligramme of chloride of rubidium 

 could not be recognized when the quantity of chloride of lithium 

 added exceeded 600 times the weight of the rubidium salt. 



At the close of this memoir we cannot refrain from touching 

 upon a question to which, on some future occasion, we must 



