186 
Dr. Thomson on some of 
without fusion. I made an attempt to determine how much of 
the acid still remained undecomposed, but did not succeed. 
Water would not separate the acid from the oxide, and fusion, 
or even digestion with an alkali could not be had recourse to, 
because it might have brought back a portion of protoxide 
to the state of acid. 
III. Brown oxide of Chromium. 
This oxide was first noticed by Vauquelin, who remarked 
at the same time that it contained only a very little more 
oxygen than the green oxide. It may be most conveniently 
prepared by passing a current of sulphurous acid gas through 
a solution of chromate, or bichromate of potash : the brown 
oxide soon precipitates, and may be easily collected on the 
filter, and washed and dried. 
It has a flea-brown colour, and is 'destitute of taste, and 
does not undergo any alteration by several days exposure to 
the air. It is not quite insoluble in water. A quantity of it 
was put into a large glass jar, which was filled with water, 
and the powder well agitated in the liquid. When the oxide 
had subsided, the water was drawn off with a syphon, and 
clean water substituted. This was continued regularly every 
day for above two months. The water always assumed a 
yellow colour, and some chromic acid could be detected, 
dissolved in it, to the very last. 
I did not succeed by this method in altering the colour of 
the oxide ; but when caustic ammonia was substituted for 
water, the colour of the oxide, after the second washing, had 
become black; and after two or three more washings, it 
