412 Messrs. Nichols and Snow on the Influence of 



been given in one of the articles on colour already 

 cited *. 



All these substances show marked decrease of reflecting- 

 power as we pass through the spectrum from red to violet. 

 In the case of sulphur, the diminution is much less rapid than 

 in other yellows until the region of the F line of Fraunhofer 

 is reached. The curve then trends sharply downwards, indi- 

 cating very rapid lessening of the reflecting-power towards 

 the violet end of the spectrum. 



The films of massicot and mercuric iodide were prepared 

 as follows : — Coatings of red lead and of the red variety of 

 mercuric iodide respectively were laid on in alcohol. When 

 these were heated by the passage of the current, they were 

 converted into the monoxide of lead (PbO) in the one and 

 into the yellow form of the iodide in the other. The change 

 took place spontaneously, when the requisite temperature was 

 reached, without any disturbance of the film, and the colour 

 of the pigment thus obtained was uniform and of uncommon 

 brilliancy. The unstable yellow mercuric iodide, however, 

 began to revert to the red form almost as soon as it became 

 cold, and measurements were obtained in four regions only. 

 The coating of molybdic acid was prepared by treatment with 

 alcohol. 



The results of our measurements of the spectra of these 

 three substances are given in Tables III., IV., and V. 



Table III. — Reflecting-power of Molybdic Acid in terms of 

 that of the ideal White. Temperatures, 25° and 272° C. 



Wave-lengths. 



Intensities at 



25° C. 



Intensities at 

 272° (J. 



fi. 



•7530 





0-720 



•6685 



0-695 



0-596 



•6080 



0613 



0-465 



•5570 



0-517 



0-335 



•4920 



0-309 



0101 



•4685 



0139 





•4500 



0-068 



0040 



•4340 



0045 





* See American Journal of Science, xxviii. p. 345. 



