420 Experiments and Olservalions 



The azure, the red and yellow ochres, and the hlacks are 

 the colours that seem not to have changed at all, in the ancient 

 fresco paintings. The vermilion is darker than recently made 

 Dutch cinnabar, and the red-lead is inferior in tint to tliat sold 

 in the shops. The greens in general are dull. 



The principle of the composition of the Alexandrian frit is 

 perfect; namely, that of embodying the colour in a composition 

 resembling stone, so as to prevent the escape of elastic matter 

 from it, or the decomposing action of the elements ; this is a 

 species of artificial lapis lazuli, the colouring matter of which is 

 naturally inherent in a hard siliceous stone. 



It is probable that other coloured frits may be made, and it 

 is worth trying whether the beautiful purple given by oxide of 

 gold cannot be made useful in painting in a densely tinted glass. 



Where frits cannot be employed, metallic combinations 

 which are insoluble in water, and which are saturated with oxy- 

 gen or some acid matter, it is evident from the proof of a du- 

 ration of seventeen centuries, are the best pigments. In the 

 red ochres the oxide of iron is fully combined with oxygen, and 

 in the yellow ochres it is combined with oxygen and carbonic 

 acid ; and these colours have not changed. The carbonates of 

 copper which contain an oxide and an acid have changed very 

 little. 



Massicot and orpiment were probably the least permanent 

 amongst the ancient mineral colours. 



Of the colours, the discovery of which is owing to the improve- 

 ments in modern chemistry, the patent yellow is much more 

 durable than any ancient yellow of the same brilliancy; and 

 chromate of lead, an insoluble compound of a metallic acid with 

 a metallic oxide, is a much more beautiful yellow than any pos- 

 sessed !)y the ancients, and, there is eveiy reason to believe, is 

 quite unalterable. 



Scheele's green (the arsenite 'of copper), and the insoluble 

 muriatic combination of copper, will probably be found more 

 unalterable than the ancient greens ; and the sulphate of baryta 

 offers a white superior to any j)os8essed by the Greeks and Ro- 

 mans. 



I have tried the eifect of light and air upon some of the co- 

 lours formed by the new substance iodine. Its combination with 

 mercury offers a good red ; but it is, I think, less beautiful than 

 vermilion, and it appears to change more by the action of light. 



Its compound with lead gives a beautiful yellow, little inferior 

 to the chromate of lead ; and I possess some of this colour which 

 has been exposed tj light and air without alteration for several 

 mouths. 



In many of the figures and ornaments in the outer chambers 



of 



