io 6 Sir Humphry Davy's experiments and observations 
larger or smaller quantities of carbonate of lime, but when 
this carbonate of lime is dissolved by acids, they present the 
same body colour, a very fine blue powder similar to the best 
smalt or to ultramarine, rough to the touch, and which does 
not lose its colour by being heated to redness ; but which be- 
comes agglutinated and semifused at a white heat. 
This blue I found was very little acted on by acids. Nitro- 
muriatic acid by being long boiled upon it gained, however, 
a slight tint of yellow, and afforded proofs of the presence of 
oxide of copper. 
A quantity of the colour was fused for half an hour with 
twice its weight of hydrate of potassa; the mass which was 
bluish green was treated by muriatic acid in the manner usu- 
ally employed for the analysis of siliceous stones, when it 
afforded a quantity of silica equal to more than | of its weight. 
The colouring matter readily dissolved in solution of ammonia, 
to which it gave a bright blue tint, and it proved to be oxide 
of copper. The residuum afforded a considerable quantity of 
alumine, and a small quantity of lime. 
Amongst some rubbish that had been collected in one of 
the chambers of the baths of Titus, I found several large 
lumps of a deep blue frit, which when powdered and mixed 
with chalk produced colours exactly the same as those used 
in the baths, and which when submitted to chemical tests were 
found to be the same in composition. 
The minute quantity of lime found in this substance was 
not sufficient to account for its fusibility : it was therefore rea- 
sonable to expect the presence of a fixed alkali in it ; and on 
fusing some of it with three times its weight of boracic acid, 
and treating the mass with nitric acid and carbonate of am- 
