on the colours used in painting by the Ancients. 107 
monia, and afterwards distilling sulphuric acid from it, I pro- 
cured from it sulphate of soda, which proves that it was a frit 
made by means of soda, and coloured with oxide of copper. 
The undiluted colour in its form of frit is used for orna- 
menting some of the mouldings detached from the ceilings of 
the chambers in the baths of Titus : and the walls of one 
chamber between the compartments of red marble, bear 
proofs of having been covered with this frit, and retain a con- 
siderable quantity of it. 
There is every reason to believe, that this is the colour 
described by Theophrastus as discovered by an Egyptian 
king;* and of which the manufactory is said to have been 
anciently established at Alexandria. 
Vitruvius speaks of it, under the name of caeruleum/f as 
the colour used commonly in painting chambers, and states, 
that it was made in his time at Puzzuoli, where the method of 
fabricating it was brought from Egypt by Vestorius ; he 
gives the method of preparing it by heating strongly toge- 
ther sand, flos nitri,£ and filings of copper. 
Pliny mentions other blues, which he calls species of sand 
(arenas) from the mines of Egypt, Scythia, and Cyprus. These 
natural blues, there is reason to believe, were different prepa- 
rations of lapis lazuli, and of the blue carbonates and arse- 
niates of copper. 
Both Pliny and Vitruvius speak of the Indian blue, which 
the first author states to be combustible, and which was evi- 
dently a species of indigo. 
I have examined several blues in the fragments of fresco 
* De Lapidibus, sect, xcviii. f Lib. vii. Cap. n. 
t This identifies the nitrum of the ancients with carbonate of soda. 
P 2 
