416 
AGATE. 
Besides this very curious circumstance in the 
agate, we meet with other peculiarities which de- 
serve our notice. Many are very prettily marked 
within with ramifications of a brown or black co- 
lour, which represent trees stripped of their leaves. 
These stones, which are called arborescent agates , 
are highly prized, and consequently much sought 
after. When the ramifications run very fine and 
numerous they are called dendrites , and are sup- 
posed to be formed by metals in a dissolved state 
which find their way between the layers of agate, 
and through imperceptible openings into the very 
body of the stone. The substances of which these 
ramifications are chiefly composed, are iron and 
manganese ; and these being purely metallic are 
easily dissolved in acids. As a proof of this, if an 
arborescent agate be dipped several times in aqua- 
fortis, the dendrites will be totally effaced, and 
merely leave a white opaque mark behind. Al- 
though agate is of a siliceous or flinty nature, and 
appears to be very compact, yet particular metallic 
solutions will not only penetrate, but colour it 
throughout. Silver dissolved in nitrous acid has this 
property in an eminent degree ; for, if a thin piece 
of agate be immersed for two or three days in this 
menstruum, and then exposed alternately to mois- 
ture and the sun for about a fortnight, the stone 
will be changed to a pretty violet colour. This 
may be again discharged by the assistance of aqua- 
fortis for twenty-four hours, and afterwards leaving 
the agate for two or three days in spring water: 
