On the Porosity of Agates, Calcedonies, §c. 169 
stones in water till it boils, and then rapidly cooling them 
(the stones must not be taken out of the water during the 
process). The heating expels a part of the expanded air 
from the cavities, through the pores, through which again 
the water is pressed on cooling, while the size of the air- 
bubble is dependent on the difference of temperature. 
In many transparent calcedonies, the little cavities which 
the stone contains may be recognised by the naked eye. 
They are seen to be small bubbles often round, often long, 
frequently running into each other, and forming tuberculous 
cavities. In others, however, they cannot be observed by 
the naked eye, but are easily seen by the aid of a micro- 
scope, under which they appear to be filled with small cavi- 
ties, especially the Brazilian Carnelian, which is particularly 
well suited for colouring. In a species of agate, which is 
called Rainbow Agate, when exposed to the sun, many well- 
known beautiful iris colours are produced. This optical phe- 
nomenon is explained by an examination of the mineral, when 
a great many small bubbles are discovered lying over one 
another longitudinally. 
The colouring of onyx and calcedonyx (if we are to under- 
stand that the white and black, or dark-brown, stones are to 
be called onyx, and the white-and-grey streaked varieties are 
to be called caleedonyx) is performed at Oberstein and Idar 
in the following way. The best stones are first well washed 
and dried without any raising of the temperature ; after this 
they are placed in honey diluted with water (one half pound 
of honey to a chopin of water). The pot in which they are 
then to be placed must be clean and free from grease. It 
must then be put into hot ashes, or into a hot oven, and the 
stones covered with the fluid, which must not be allowed to 
boil. The minerals must, indeed, always be covered with the 
fluid, which must be added from time to time. The minerals 
are to be treated in this way for a fortnight or three weeks. 
They are then taken out of the honey, washed, and placed 
in another vessel along with as much oil of vitriol as will 
cover them. The vessel is then to be covered with a lid, 
and placed in ashes in which hot coals are placed. The 
porous or soft stones are coloured in an hour, others in a day, 
