432 On the Changes of Colour 



Experhnent 4. — Nitrate of Cobalt slightly dried. 

 This salt, which when cold is of a wine red, becomes 

 blue the instant its temperature is a little raised : when 

 cooled it resumes its primitive colour, and thus passes suc- 

 cessively from one tint to another, when we vary its tem- 

 perature, independently of the influence of humidity. 



Experiment 5. — Red Sulphuret of Jrsenic, or Realgar . 



When in the mass it is red : but when it is pounded it 

 has an orange colour mixed with a chesnut red. On ex- 

 posing it to heat, it takes the colour of colcothar. 



Experime?it 6. — Glass of Ayitimony . 

 Tt presents when pounded an orange yellow colour 

 clouded with a good deal of grav. At a heat of about 400* 

 it as-^umes a brownish red, as if it had been mixed with the 

 rtd oxide of iron. 



Experiment 7- — Oxide of Bismuth prepared, hy decomposing 

 the Nitrate of Bismuth by Heat. 

 Its colour is a dirty white mi.xed with a little orange yel- 

 low. On eraduallv raising its temperature, it becomes of 

 a very fine yellow, and passes progressively to the chesnut 

 red : upon cooliiia it resumes its primitive colour. We 

 ought to remark, that it does not pass by the pure orange: 

 thus the red tint which it acquires is not a pure red, but 

 that of the rust of iron. 



Experiment 8. — Oxide of Tin prepared with Nitric Acid, 

 and calcined. 



Its colour is similar to flowers of sulphur sullied by a 

 little grav. When heated it assumes a yellower shade with 

 a little red. 



Experiment Q. — Oxide of Zinc. 



Upon calcining nitrate of zinc exempt from iron, an 

 oxide was obtained which when cold is of a straw colour: 

 a heat of about 300° gives it a colour like that of Naples 

 yellow. At a higher temperature the shade is yellower, and 

 compared with that of the chromate of lead is greener, and 

 more intense. 



Experiment 10. — Flowers of Sulphur. 



The first stage of heat produced a more lively yellow, in 

 which we remarked the gray tint which accompanies this 

 colour. When they began to fuse, the shade became olive 

 yellow, and the colour increasing it became red. 

 Experiment 1 1 . — Yelloiv Sulphuret of Arsenic, or Orpiment. 



We may regard this colour as the purest which mineral 



substances 



