362 
ON OXIDE OF ZINC. 
the powder, and other portions are transferred to the fundus of the 
flask, while the aqueous vapor is more quickly and abundantly 
disengaged and deposited on the neck. In order to remove this 
the more readily, a pair of small hand-bellows may be employed 
for the purpose of gently blowing into the neck of the flask. After 
repeated shaking the process is terminated. Its completion may 
be known by the circumstance of the powder no longer floating, 
as it were, on the surface of the stratum of vapor, but subsiding to 
the bottom of the vessel. A proof of the absence of carbonic acid 
may be found by placing the powder in a porcelain dish, and re- 
peating the operation with a fresh quantity.* 
The experiment and testing are more easily performed in a 
porcelain dish, as the shaking can therein be more exactly regu- 
lated. If the temperature be raised too high, the powder will be 
burnt yellow even under a red heat. This observation of the yel- 
low discoloration from a pure white by irregularity of the heat, 
has led Dr. Wittstein to the consideration of the question, often 
discussed, of the cause of the yellow color of oxide of zinc pre- 
pared in the moist way. For this purpose he has instituted a series 
of experiments, and has shown, among other conclusions, that the 
adhering sulphate or alkali exerts an essential influence, although 
at present the question cannot be fully answered. 
Under these circumstances a theory is advanced by the" author 
which may be considered as not entirely absurd. We know that 
the estimate of the color of a body is different according as it is in 
a state of minute division, or in that of a compact solid form. 
Thus brown iron ore gives a dirty yellow powder ; and a brownish 
piece of gum Arabic will produce a perfectly white powder ; and 
so forth. It is extremely seldom that the reverse is noticed, viz., 
that the powder is darker than the mass ; such is the case with the 
colorless diamond, which gives an ash-grey powder. The author 
is of opinion that the change of white oxide of zinc to that of a 
yellowish white, from increase of temperature, may probably be 
owing to a change in the state of its molecules, somewhat re- 
sembling that of incipient fusion ; and he hoped to have been able 
to have determined this point by the help of the microscope. 
* In the same manner, Mohr converts carbonate of magnesia into pure 
magnesia. 
