Temperature on the Colour of Pigments. 



419 



of the other pigments studied. It is interesting to compare 

 it in this respect with ultramarine (fig. 13) , in which there 



Fig. 13. 



is a well-defined maximum in the blue, which is cut down 

 in heating without any lateral displacement of the curve. 



Both chromic oxide and ultramarine had been measured 

 in the course of the earlier investigation on the spectro- 

 photometry of pigments, to which reference has already been 

 made. A comparison of the curves then published with 

 those plotted from the data contained in Table XI. shows the 

 ultramarine used in the original experiments to have been of 

 very nearly the same brilliancy in the blue as the later 

 specimen. The chromic oxide, however, was more than three 

 times as bright in the green as that to which the quantities in 

 Table XI. refer. 



The series of experiments w r hich have been described in- 

 cluded also measurements of lamp-black, hot and cold, and of 

 the oxide of zinc. 



Our purpose in the study of lamp-black was simply to 

 determine whether this substance, like the other pigments 

 subjected to measurement, decreased in reflecting-power with 

 rise of temperature, and if so, to what extent. 



2 F 2 



