334 F. JB. Pitcher — Absorption Spectra of Blue Solutions* 



from the second Argand, and the intensity ratio between the 

 spectrum thus transmitted and that of the comparison flame 

 was obtained for nine regions, lying between the A line and 

 the Gr line of Fraunhofer. 



The wave-lengths of these regions, were as follows : 



Region. Wave-length. 



1 7670 



2 ___ 6770 



3 6150 



4 5630 



5 . 5235 



6 4940 



7 4720 



8 4525 



9 4360 



These readings afforded a correction -factor for each region of 

 the spectrum, by means of which the selective absorption due 

 to the glass cell and the condensing lens could be eliminated. 



Solutions were then made of a number of blue pigments. 

 Three of these were of known composition, namely, Prussian 

 blue, artificial ultramarine and indigo. Five others were com- 

 mercial preparations of " bluing " of unknown composition. 

 These were included in the list, at the writer's suggestion, for 

 the purpose chiefly of testing the applicability of the spectro- 

 photometric method to the detection of such coloring matters 

 as are not amenable to the ordinary spectroscopic analysis. 



The strength of the solutions was so chosen in each case as 

 to show decided color, and at the same time to transmit easily- 

 measurable quantities of light throughout the spectrum. The 

 results were indicated by means of curves, wave-lengths being 

 taken as abscissae, percentage of light as ordinates ; that trans- 

 mitted by the empty cell in each region being taken as unity 

 for that particular region. 



Figure 2 shows the curves thus obtained for ultramarine 

 (curve a), indigo (curve b) and Prussian blue (curve c). They 

 exhibit the nature of the selective absorption excited by each 

 solution upon the rays traversing it. The horizontal line at the 

 top of each diagram may be considered as representing the 

 brightness of the spectrum of the rays passing through the 

 empty cell, as compared with that of the light transmitted by 

 the solution under investigation. Yertical distances from that 

 line to the curve measure the degree of absorption in each por- 

 tion of the spectrum. Each curve is characteristic of the solu- 

 tion to, which it pertains and, in so far as the number and posi- 

 tion of the maxima and minima are concerned, it is inde- 

 pendent of the strength of the solution. 



These results afford an excellent illustration of the use of 

 the spectro-photometer in the location of absorption bands not 



