10 
Field Museum of Natural History 
dipped into a 25% solution of silver nitrate draw a 
band of silver nitrate about the outer edge of the gela- 
tine. Put the plate in a cool, dry place not exposed 
to sunlight and watch the course of the reaction. 
Gradually the silver nitrate will penetrate the gela- 
tine and the formation of bands of silver chromate 
will begin. The process will continue and bands be 
formed for 24 to 48 hours or until the silver nitrate 
and ammonium bi-chromate have exchanged positions, 
the silver nitrate ultimately reaching the center of 
the plate and the ammonium bi-chromate the circum- 
ference. 
The formation of bands of this sort was first made 
known by a German chemist, Raphael Ed. Liesegang, 
and the phenomenon is now generally known as "Liese- 
gang's rings." The width of the bands and other 
features can be varied by using other percentages of 
chemicals than those given here and other chemicals. 
Then too, by varying the points of application of the 
silver nitrate or by introducing it as drops or lines, 
imitations of other patterns seen in agates can be 
obtained. 
According to the present view, then, fortification 
or common agates originate from a filling of hollow 
spaces in rocks by a silica colloid (jelly), within which 
an iron compound has been rhythmically deposited. 
Although the banding of fortification agates can 
be acceptably explained by the introduction of iron 
salts into colloidal silica, it is at first sight difficult to 
see how moss and pipe agates and some other forms 
can be accounted for in this way. 
For a long time and even in mineralogical text 
books up to the 19th century it was thought, and stated, 
that moss agates were made up of moss enclosed in sili- 
ca. Peculiar enclosures in some other kinds of agates 
were also explained as petrifications of other once living 
[114] 
