394 Gooch and Phinney — Quantitative Determination 



immediate volatilization of the chloride so increased the deli- 

 cacy of the spectroscopic reaction that it was possible to see 

 the lines from 0*00005 mg. of the salt. These figures serve as 

 an indication of the possible delicacy of this method of pro- 

 ducing spectra, hut it should be remembered that all eyes do 

 not see the rubidium lines with equal ease. 



In experimenting with rubidium sulphate in place of the 

 chloride the lighter coil appeared to lose its advantage in point 

 of delicacy of the indication over the larger coil, possibly be- 

 cause the amount of metal introduced into the flame has less 

 influence upon the rate of vaporization of the less volatile sul- 

 phate, and the lines of the sulphate appeared equally distinct 

 and of longer duration than those of the chloride ; but, while 

 we judged the sulphate to be rather preferable in qualitative 

 work, our earlier preliminary quantitative experiments led us 

 to abandon it for the purposes of our investigation on account 

 of the great uniformity in the lines yielded by the chloride. 

 We found it desirable in comparative tests of brightness to 

 employ as the standard the lines given by amounts of the 

 chloride not exceeding 0"0005 mg. to 0*0007 mg., to set the 

 slit at a width of 0*2 mm , and to bring the coils to the flame in 

 sets of three — the first, usually a standard, serving to fix the 

 position of the lines so that the comparative distinctness of the 

 lines given by the other two might be the more readily deter- 

 mined. The tables below contain the record of an attempt 

 to dilute a solution of the pure chloride to the strength of the 

 standard solution under the guidance of comparative tests of 

 the brilliance of the spectral lines yielded by the residue left 

 upon the wires after the evaporation of the solutions taken up 

 by coils of equivalent capacity. Dilutions were made in ordi- 

 nary graduated cylinders of suitable capacity. 







Experiment 



I. 





Standard. 



Test (known 



to 



Line of test 



Rubidium iu 



contain 10 mg 



Rb.). 



compared 



a coil-full 



Uo cm 3 ). 



Volume in cm 3 . 



with standard. 



0*0005 



mg. 



200 





Brighter. 



« 



u 



300 





Brighter. 



a 



u 



350 





Brighter. 



a 



a 



370 





Brighter. 



a 



(i 



390 





Brighter. 



a 



a 



400 ) 





f Brighter (faintly). 



a 



• c 



400 [■ 





-j Brighter (faintly). 



a 



a 



400 ) 





f Doubtful. 



a 



u 



410 j 





j Brighter (faintlv). 



a 



a 



410 \ 





1 Weaker. 



a 



a 



420 j 





j Equally bright. 



a 



a 



420 \ 





| Weaker. 



a 



a 



430 





Invisible. 



