Temperature on the Colour of Pigments, 



423 



Table XIII. — Reflecting-power of Zinc Oxide in terms of 

 that of the ideal White. Temperatures (A), 25°, 578°, 

 750°, and 815° C. (B) 25° and 600° C. 







(A) Intensities at 





(B) Intensities at 



Wave- 

 lengths. 



























25°. 



578°. 



750°. 



815°. 



25°. 



600°. 



fl. 



•7530 



0-675 



0-564 





0769 



0-654 



0-797 



•6685 



0-779 



0-734 



0-793 



0-854 



0-794 



0-685 



•5570 



0-705 



0-759 ! 0-712 



0650 



0-691 



0-587 



•4920 



0-749 



0-707 



0569 



0-549 



0-727 



0-532 



•4500 



0-620 



0-501 



0-238 



0-150 



0-598 



0-347 



•4250 



0-486 



0-178 



0049 



0-049 



0-470 



0-076 



In these experiments we investigated comparatively few of 

 the long list of substances given by Houston and Thomson 

 as subject to colour-change by heating, or of those catalogued 

 by Ackroyd as " m eta-chromic." A sufficient number were 

 tested, however, to make it clear that colour-change by 

 temperature is a very general phenomenon ; also that the 

 statements of Schoenbein, and of the later authors just men- 

 tioned, are accurate, in so far as their methods enabled them 

 to observe the facts. 



Our results may be summarized as follows : — 



1. None of the pigments tested equals the ideal white in 

 reflecting-power, even in that part of the spectrum for which 

 its reflecting-power is greatest. 



2. The reflexion spectrum of pigments arises from two dis- 

 tinct sources: (a) light reflected from the surface of the 

 substance ; (b) light reflected from interior faces. The light 

 reflected from the surface is nearly white. Its brightness 

 varies from about 2 per cent, (as in HgS) to nearly 10 per cent, 

 (as in HgO). It is to the light internally reflected that the 

 pigment owes its colour. 



3. The effect of heating a pigment is invariably to diminish 

 its reflecting-power, the diminution being as a rule more 

 marked in regions of greatest refrangibility. 



4. The changes of colour, observable when a pigment is 

 heated, are due to this unequal loss of reflecting-power, and 

 the effect which has been described as "a shifting of the 

 colour towards the red/'' arises from the fact that the loss of 

 brightness is least in the red and increases rapidly as we pass 

 towards the violet end of the spectrum. There are cases, 



