Temperature on the Colour of Pigments. 



417 



Table IX. — Reflecting-power of Ferric Oxide (Fe 2 3 ) in 

 terms of that of the ideal White. Temperatures, 25°, 



493°, and 687° C. 



Wave-lengths. 



Intensities 



Intensities 



Intensities 



at 25° C. 



at 493° C. 



at 687° C. 



fl. 



•7530 



0-308 



0-113 



0-123 



•6685 



0-180 



0*072 



0070 



•6080 



0095 



041 



0-044 



•5570 



0-026 



0-035 



0-035 



•4920 



0042 



0042 



0046 



•4500 



0-029 



0-030 



0-024 



Our study of green and blue was a very cursory and in- 

 complete one. It was confined indeed to the examination of 

 chromic oxide and " artificial ultramarine/' The former was 

 of a very dull green colour. It was the usual preparation of 

 the chemical laboratory, and not the more brilliant variety 

 known as chrome-green. 



On account of the very small amounts reflected by these 

 pigments, the measurement of their spectra was attended with 

 some difficulty. The results obtained, for which the same 

 degree of accuracy as in the case of the foregoing deter- 

 minations is not claimed, are given in Tables X. and XI. 

 In the accompanying diagrams abscissae are on the usual 

 scale, but ordinates are increased five-fold. 



It was evident to the unaided eye that the result of heating 

 the ultramarine was to alter its hue permanently. A few 

 measurements upon the cooled pigment were therefore made 

 to determine the amount of the change. The results, which 

 indicate very marked alteration in the colour of the substance, 

 are given by themselves in Table XI. 



Table X. — Reflecting-power of Chromic Oxide (Cr 2 3 ) in 

 in terms of that of the ideal White. Temperatures, 25°, 

 360°, and 730° C. 



Wave-lengths. 



Intensities 

 at 25° C. 



Intensities 

 at 360° C. 



Intensities 

 at 730° C. 



•7530 



0-058 



0-042 



0-059 



•6685 



0-062 



0-054 



0-070 



•6080 



0056 



0-078 



0070 



•5570 



0-098 



0-069 



0-054 



•5185 



0-073 



0-044 



0037 



•4920 



0-046 



0-038 



0-028 



•4500 



0-029 



0-032 



0017 



Phil. Mag. S. 5. Vol. 32. No. 198. Nov. 1891. 2 F 



