Agate— Physical Properties and Origin 13 
pages of this leaflet that this view is incorrect. At any 
rate modern methods of coloring agates were not intro- 
duced until 1819, when a method of coloring black was 
accidentally discovered in Germany. Production of a 
red color by burning was to be seen in nature, since 
one could there observe that portions of agates pro- 
truding from the earth were reddened as compared 
with the covered portions. This natural process was 
at first imitated by exposing agates to the heat of the 
sun, but later the same result was obtained more 
quickly by heating the stones in ovens. It being 
found, however, that some stones would not color in 
this way, it was concluded it was because they con- 
tained no iron compounds. Accordingly experiments 
were made to introduce iron compounds into the stones, 
and the colors were then successfully obtained. Hav- 
ing succeeded in this, the production of other colors 
was sought. In 1845 a method of coloring blue was 
discovered and in 1853 one of coloring green. Dreher 
in his book on the coloring of agates gives the follow- 
ing table of colors which can be produced in agates 
and the methods of producing them : — 
DESIRED 
COLOR 
Red 
Bluish-green 
Apple-green 
Brown 
Blue 
Blue 
Black 
COLORING 
MATTER 
Iron oxide 
Chromic oxide 
Nickel oxide 
Caramel 
Berlin blue 
Turnbull's blue 
Carbon 
HOW 
OBTAINED 
From iron nitrate by heating. 
From chromic acid or am- 
monium bi-chromate by 
heating. 
From nickel nitrate by heat- 
ing. 
From sugar by strong heat- 
ing. 
From yellow prussiate of 
potash and iron vitriol. 
From red prussiate of pot- 
ash and iron vitriol. 
From sugar and sulphuric 
acid. 
[117] 
