204 Mr Purvis, On a method of estimating the amounts ■ 



movable cylinder X to the internal lower end of the cylinder G 

 are exactly the same as the bands observed in the solution in B, 

 the concentrations of the solutions must be inversely as the 

 thickness producing the absorptions, and that between these 

 limits there must be the same amount of the oxide dissolved in 

 the one as in the other. Further, as the amount of the oxide 

 dissolved in any aliquot portion of the solution in G is known, it 

 is easy to read off by the scale the amount of the oxide dissolved 

 in the solution in B. 



It is desirable not to have the solutions too highly concen- 

 trated, because the variations in the intensities and widths of the 

 various bands are not so easily distinguished when the solutions 

 are too strong. It was found to be convenient to observe, in the 

 Didymium solutions, the three bands in the blue part of the spec- 

 trum at about A, 483, A, 476 and A469; also the bands in the green 

 at about A 51 9 and A 522 ; and particularly the bands in the yellow 

 at about A570 — A, 577. When the solutions are made of such 

 strengths that these bands are distinctly and separately seen, 

 the experimental results were found to be very exact; whereas 

 when the solutions are very strong the bands in the green and 

 yellow parts of the spectrum close up and appear very broad and 

 black, causing the delicacy of the comparison to be very much 

 lessened. In the solutions of Erbium, the bands at A536, A 541, 

 A 523 and A 488 were found the best to observe and compare and 

 for the same reasons. 



It is possible that, if photographs were taken, the adjustment 

 of the movable cylinder X could be made much more accurately, 

 and therefore an increased accuracy of the results obtained. 



It is obvious that if both oxides of Didymium and Erbium 

 are in solution in B, it is only necessary to place in the side tube G 

 definite volumes of definite strengths of the solutions of each of 

 these earths, and compare the bands which are recognized to 

 belong to either Didymium or Erbium. 



The method can, of course, be used for any solutions which 

 give absorption spectra, and which are not altered on dilution. 



Some results obtained by the method. 



The following tables give some of the results obtained by this 

 method. The tube B had an internal volume of 31*5 c.c, and was 

 filled to the lower level of the side tube Y with the solution to be 

 investigated. A definite volume of a definite strength of the 

 solution of the salt was placed in the tube G, and the movable 

 cylinder X adjusted until the bands of the two parallel spectra 

 were identical. The scale on the tube G was read, and the 



