On the Porosity of Agates, Calcedonies, &c. 7h 
Calcedony can be coloured a very fine citron-yellow, either 
generally diffused or streaked (if this condition is already in- 
dicated in the stone). The process is as follows: they are 
first dried two days in an oven, care being taken not to let 
the oven become too warm; the stones are then to be placed 
in a clean vessel, and covered with spirit of salt. A cover 
must be firmly cemented on the vessel with clay ; they must 
then remain from fourteen days to three weeks in the oven, 
and then the yellow colouring process is complete. 
It deserves further inquiry, whether this yellow colour is 
occasioned by the formation of a salt,--by the mixture of the 
hydrochloric acid with some previously existing matter in the 
stone itself, or whether the colouring principle is entirely 
contained in the acid. I know no natural calcedony having 
a colour similar to that produced in this way. There occurs, 
however, in opals such a citron-yellow colour, but it is rather 
more of the appearance of wax. 
In the coloured stones, however, this shade shews itself 
here and there, and seems to be inherent in them, as the 
colouring matter always remains the same. 
Of late years a very fine blue colour has been produced in 
calcedony, shewing all the different shades of the torquoise. 
The process for this is yet a secret, known only to a few of 
the cutters. 
Many minerals are also burned,—such as agate, calcedony, 
and Brazilian carnelian. This is done partly to increase the 
beauty of their natural colours, and partly, as it is said, to 
give the natural colours more durability. Many calcedonies 
become, through this process, almost white, the red colours 
more intense, and the pale yellow a very fine red. This is 
also the case with the Brazilian carnelian. By which pro- 
cess the streaked stones of this kind become transformed 
into fine sardonyxes, and those with one colour take on the 
true colour of the carnelian. The process is as follows: 
the stones are rendered perfectly dry, by being placed for a 
fortnight or three weeks in a hot oven; they are then placed 
in a shallow dish and moistened with vitriolic acid, but not 
covered. The polishers usually dip the stones in the acid, 
