of Chemical Compounds. 



131 



will be well to state briefly the more important results which 

 have been obtained by other investigators in regard to the 

 first and second. 



I. Influence of Temperature. — As to the influence of tem- 

 perature on the colour of chemical compounds, it appears, as 

 shown more particularly by Mr. Ackroyd, that many bodies 

 when heated undergo alterations in colour ; these alterations 

 being such that, as the temperature rises, the colour passes 

 through the following chromatic scale: — 



White or Colourless. 



Violet. 



Indigo. 



Blue. 



Green. 

 Yellow. 



Heating. 



Cooling 



Orange. 

 Eed. 

 Brown. 

 Black. 



In oiher words, nearly all chromatic changes take place in a 

 definite order, viz. the order of the spectrum colours, in such 

 a way that, as the temperature rises, the colour passes more 

 and more towards the red end of the spectrum ; and subse- 

 quently, if the temperature be high enough, to brown and 

 black. Most commonly the colour passes directly from white 

 to pale yellow, the violet, indigo, blue, and green stages being 

 omitted. As examples we may take the following: — ■ 



Colour assumed as the 

 temperature rises. 



white yellow and then orange. 



red dark red. 



„ 140° C. vellow (in sealed tube) orange and then red. 

 PbO vellow ' 



Normal colour. 



ZnO 



HgI 2 ,16°C. 



Til 



r ellow 



Cu(B0 2 ) 2 ... blue 



PbCrO, 



orange 



orange and then red. 



orange -yellow and then 

 orange-red. 



green and then greenish- 

 yellow. 



brick-red and then black- 

 red. 



orange, then red, and 

 finally dark red. 



orange, then red, and 

 finally brown. 



II. Influence of the Quantity of the Electronegative Element 

 in Binary Compounds. — It has been pointed out by Ackroyd 

 and by Petrie that, in the case of binarv compounds, an 



K2 



Agl yellowish-white 



HgO orange-yellow 



