ON OXIDE OF ZINC. 
363 
With this view a portion of carbonate of zinc, which had been 
ascertained by the usual tests to be free from any trace of iron or 
other metallic impurity, was placed in a porcelain dish over a 
spirit-lamp, and constantly stirred until it became so hot that a 
test-portion added to dilute sulphuric acid gave off no bubbles of 
gas — a point which speedily occurs. This preparation had the 
same snow-white color as the pure carbonate. Placed under a 
microscope, having a magnifying power of one hundred and twenty, 
the separate 'particles of the powder were easily observed ; under 
this power they appeared as globules of about the size of pins' 
heads, frequently smaller, seldom larger, never exceeding l-140th 
of a line in diameter : their form was changed by moisture. The 
parts that were moistened were distinguished from the surrounding 
powder by a distinct pale yellow color. 
The oxide was now heated for a quarter of an hour ; when quite 
cold it was no longer of snowy white but milk white ; the differ- 
ence could readily be appreciated by a comparison w T ith the pure 
oxide. Magnified a hundred and twenty times, the particles ap- 
peared opaque, smaller than before, and often arranged in a string- 
like form. There were also observable numerous yellowish-brown 
particles dispersed among the yellowish-white mass. When mag- 
nified three hundred times the globules were transparent — the 
oxide remained the same after it had been heated half an hour, one 
hour, and two hours. Between a quarter of an hour and two 
hours the oxide undergoes absolutely no change, neither to the 
naked nor to the assisted eye. 
The author considers that he has thus made an approach to- 
wards the solution of this riddle, although the above theory 
should not be confirmed or should even be controverted. There 
can be no doubt that the yellow discoloration retained by the 
oxide after cooling is owing to the yellowish-brown particles 
scattered throughout the mass. It still remains to discover the 
nature of these granules in order to answer this long-debated 
question of the cause of the yellow color of the oxide of zinc pre- 
pared by heat.' 
It has been above observed that the carbonate of zinc operated 
upon had. been found pure by the usual reagents. By testing, it 
was examined for iron, lead, cadmium, and copper ; the acidulated 
solution of the hydrochlorate with sulphocyanuret of potassium, 
