The blue colour of the sky, and the name given to a celebrated Egyptian pigment, which has preserved its brilliancy of tint for upwards of seventeen hundred years. The late Sir Humphrey Davy, who experimented on the constitution of this substance, found that it might be easily and cheaply imitated. Thus take 15 parts of the carbonate of soda, 20 of powdered opaque flints, and 3 of copper filings; strongly heat them together for two hours, when a substance will be produced, which, when powdered, affords a fine deep-coloured blue pigment, closely resembling the Egyptian azure.