OBSERVATIONS ON ACETATE OF ZINC. 243 
Whilst on this subject it may be well to make a few re- 
marks touching other points in the manufacture of this salt, 
for the benefit of those who may not have learned them by 
experience. 
1st. The large amount of zinc directed in the Pharma- 
copoeia is not necessary when the metal has a great surface 
in proportion to its weight ; as when in the form of sheet 
zinc. 2d. It sometimes happens that the precipitation of 
the lead proceeds very slowly, which is owing to the surface 
of the zinc being coated with oxide or other foreign matter. 
By washing the zinc with diluted acetic acid, before intro- 
ducing it into the solution, it acts very promptly. 3d. As 
in large operations it is of much importance to employ as 
little fluid as the nature of the case will admit, in order to 
save evaporation, it has been found that by mixing the 
sugar of lead with one-half of the prescribed quantity of 
water, the excess of it is gradually dissolved and decomposed 
as the operation proceeds, because the resulting acetate of 
zinc is less bulky and more soluble than the lead salt. 4th. 
In effecting the crystallization of acetate of zinc, it frequent- 
ly happens that instead of brilliant six-sided plates or tables 
of large size, the salt is deposited in minute scales or crys- 
tals, radiating from centres which form a thick magma with 
the mother liquid, difficult to drain. This is owing to con- 
centrating the solution too much, or to cooling it too rapidly, 
or both. By adopting Faraday's suggestion of removing a 
drop to a glass plate from time to time, as the evaporation 
proceeds, the proper point of concentration is easily ascer- 
tained. Sometimes, however, this imperfect crystallization 
is owing to the presence of a less soluble sub-salt, (caused 
by the loss of acetic acid) which is known by the opacity of 
the liquid when an attempt is made to re-dissolve the ace- 
tate. In order to remedy this it is necessary to add as much 
acetic acid to the hot concentrated solution before crystal- 
lization, as will enable that solution to retain its transpa- 
rency when added to twice its bulk of water. 
