Note 249 — ESSAYS. (continued 3)

Sir Humphrey Davy, in his paper in the first part of the volume of the Philosophical Transactions for 1815, on the colours used in painting by the ancients, says, that the artificial blue found in the baths of Titus is a frit made by means of soda, and coloured with oxide of copper. He imagines it to have been the blue invented by an ancient king of Egypt mentioned in the text, and the same also with that cœrulium, the art of making which was brought from Egypt to Puteoli, by Vestorius. That was made by heating together sand, flower of nitre, i. e. soda, and filings of copper.