io8 Sir Humphry Davy’s experiments and observations 
painting from the ruins near the monument of Caius Cestius. 
In a deep blue approaching in tint to indigo, I found a little 
carbonate of copper, but the basis of this colour was the frit 
before described. 
The blues in the Nozze Aldobrandine, from their resisting 
the action of acids, and from the effects of fire, I am inclined 
to consider as composed of the Alexandrian or Puzzuoli blue. 
In an excavation made at Pompeii, in May 1814, at which 
I was present, a small pot containing a pale blue colour was 
dug up, which the exalted personage, by whose command the 
excavation was made, was so good as to put into my hands. 
It proved to be a mixture of carbonate of lime with the Alex- 
andrian frit.* 
Vitruvius states, that the ancients had a mode of imitating 
the Indian blue or indigo, by mixing the powder of the glass 
called by the Greeks vuXog, with selinusian “ creta” or annu- 
larian “ creta”, which was white clay or chalk mixed with 
stained glass ; the same practice is likewise referred to by 
Pliny. 
There is much reason for supposing that this stained glass, 
or CaXog, was tinged with oxide of cobalt; and that these colours 
were similar to our smalt. I have not found any powdered 
colour of this kind in the baths of Titus, or in any other Roman 
ruins ; but a blue glass tinged with cobalt is very common in 
those ruins, which when powdered forms a pale smalt. 
I have examined many pastes and glasses that contain oxide 
of copper ; they are all bluish green, green, or of an opaque 
watery blue. The transparent blue glass vessels which are 
• This probably is the same colour as that examined byM. Chaptal. He did not 
search in it for alkali, or there is every reason to suppose he would have found soda. 
