122 S ir Humphry Davy’s experiments and observations 
provements in modem 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 possessed by the ancients, and, there is every 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 possessed by the Greeks 
and Romans. 
I have tried the effect of light and air upon some of the 
colours 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 infe- 
rior to the chromate of lead ; and I possess some of this colour 
which has been exposed to light and air without alteration 
for several months. 
In many of the figures and ornaments in the outer chambers 
of the baths of Titus, where only outlines or spots remain, or 
shades of ochre, it is probable that vegetable or animal colours, 
such as indigo and the different dyed clays, were used.* 
Pliny speaks of the celebrated Greek painters as employ- 
ing only four colours. “ Quatuor coloribus solis immortalia 
ilia opera fecere : ex albis Melino, ex silaceis Attico, ex rubris 
* Some excellent pictures have suffered very much in modern times from the same 
cause; the lakes in the frescos of the Vatican have lost much of the brilliancy which 
they must have possessed originally. The blues in many pictures of Paul Veronese 
are become muddy. 
