F. B. Pitcher — Absorption Spectra of Blue Solutions. 335 



sufficiently marked to be identified by ordinary inspection, and 

 in the detection of regions of maximum brightness in spectra 

 which to the unaided eye do not exhibit any such peculiarity. 



The measurement of the iive commercial blues gave curves, 

 which approached so nearly to those obtained in the case 

 of the three pigments already mentioned, as to leave no ques- 

 tion as to the nature of the substance to which each of them 

 owed its color. Indeed the ease and certainty with which the 

 pigment could be identified was such as to warrant the belief 

 that by the use of this instrument a large number of absorption 

 spectra, which are quite beyond the reach of the spectroscopic 

 methods now in vogue, may be identified with the same degree 

 of certainty which how attends the recognition of blood or 

 chlorophyl. To this end the first step must be the careful deter- 

 mination of a large number of characteristic curves pertaining 

 to the various solutions to which the method is to be extended. 

 This having been properly done, the identification of solutions 

 in which selective absorption takes place to a measurable extent, 

 will be in most cases a comparatively simple matter. 



' Mr, Pitcher's* experiments were also extended to two inter- 

 esting solutions in which the addition of an alkali produces a 

 change of color. These, which represent quite distinct classes, 

 were litmus solution and the 2 . 



sulphate of copper. The re- 

 sults are shown in figures 3 

 and 4. The litmus solution 

 was taken in its three charac- 

 teristic conditions, acid, alka- 

 line and neutral. The acid 

 solution (figure 3, curve d), 

 is marked by almost com- 

 plete transparency to rays of 

 the regions between B and 

 C, and to slight minima in 

 the neighborhood of the D 

 line, and just beyond F. 

 The addition of ammonia, 

 until the well known color- 

 change indicative of neutral- 

 ity took place, brought about 

 a modification of the curve 

 to the form shown in curve 

 e. The bright region in the 

 red disappeared ; the absorp- 

 tion in the yellow was in- 

 creased, and the brightness 

 of the remainder of the spec- 

 trum was diminished, by 



G 



