OXIDE OF COBALT AND MAGNESIAN OPAL. 171 



any form whatsoever, and the only one where it exists in the 

 form of an oxide free from arsenic. 



Magnesian Opal prom near Harmanjick, Asia Minor. 



I obtained for the composition of this opal, which occurs 

 with serpentine and carbonate of magnesia, silica, 92.00 ; water, 

 4.15; magnesia, 3.00. It occurs also on the island of Mytilene. 



