Temperature on the Colour of Pigments. 



409 



Table I. — Keflecting-power of Magnesium Oxide at 25° and 

 758°, in terms of that of the ideal White. 





Temperature < 



)f the pigment : 



Wave-lengths. 









25° C. 



758° O. 



H. 







•7530 







•6685 



1-021 



0-988 



•6080 



0-994 





•5570 



0-980 



0-966 



•5185 



0-949 



0-892 



•4500 



0-806 



0-737 



•4250 



0-614 



0-5843 



First on our list of substances from which marked changes 

 of colour under the action of heat might be looked for was 

 sulphur. Every one is familiar with the increase in the inten- 

 sity of the yellow hue when this substance is heated to tem- 

 peratures in the neighbourhood of its melting-point. Our 

 interest had been particularly aroused, however, by a state- 

 ment of E. Wiedemann's to the effect that, when exposed to 

 the intense cold obtained by contact with solid carbon dioxide, 

 sulphur is not yellow, but white. Professor Wiedemann* 

 alludes to this as to a well-known fact ; but it was quite new 

 to us, and we repeated the experiment, extending it to various 

 other pigments, with results which will be described later. 



For the purpose of the determination of the spectrum of 

 sulphur, a layer of the powdered element was prepared by the 

 method of moistening with alcohol, already described. When 

 the film was dry, it was measured at 25° and at 103° 0. The 

 results are given in Table II. and in fig. 4. 



Table II. — Keflecting-power of Sulphur at 25° and at 103°, 

 in terms of that of the ideal White. 



Wave-lengths. 



Intensities 

 at 25° C. 



Intensities 

 at 103° C. 



•6685 



0-682 



0-457 



•6080 



0-651 





•5570 



0-634 



0-368 



•4920 



0-571 



0-332 



•4685 



0-318 



0-205 



■4500 



0-163 



0-094 



•4340 



0055 



0-040 



The spectrum -curve of sulphur (fig. 4) is characteristic ; 

 * Beiblatter zu den Annalen der Physik, xii. p. 791. 



