MANUFACTURE OF ACETATE OF SODA. 
147 
is most convenient where it is desirable to economise room ; each 
ought to communicate with both cisterns, so that when one back 
has ceased to pour strong liquor into one cistern the pipe may be 
stopped with a plug, and that communicating with the other cistern 
opened for receiving the washings of the backs. It may be ascer- 
tained when the precipitate is sufficiently washed by the taste of 
the liquor running away from it into the weak liquor cistern, when 
it is tasteless or nearly so, the sulphate of lime must be dug off 
with a wooden spade or shovel (iron is apt to cut the flannel), the 
back is then ready for another batch from the decomposing vessel. 
The liquor is now to be pumped from the cisterns into the iron 
pans, which ought to be so cast that they may offer as much heated 
surface as possible, and yet be not too large for a man to keep their 
contents in a state of brisk agitation with an iron stirrer broad and 
flat at the end. As it approaches dryness the temperature should 
not much exceed 500°, otherwise the acetate will be decomposed 
and converted into carbonate. Great nicety is required in this part 
of the manipulation, which depends entirely on the skill of the 
workman. When the mass is in quiet fusion, and there is no 
frothing up, the process is usually finished. Having thus dissipa- 
ted the tar, the dry acetate may either be at once converted into 
acetic acid, or dissolved and crystallized ; it is more soluble in the 
latter condition. It is most conveniently dissolved in a large cylin- 
drical lead vessel, heated by shooting steam into it. When the 
solution is completed, it may be run through a flannel filter into 
the top of a course of steamers, furnished with coils of three-quarter 
inch lead pipe. These vessels are made of four pound sheet lead 
cased in boards. The best size is about twenty-four feet long, four 
feet wide, and about nine inches deep. The pipe should be coiled 
from one end to the other, and go up and down about three times. 
Too much pipe cannot be used, as the rapidity of the evaporation 
depends upon the quantity employed. As the evaporation proceeds, 
the liquor ought to be syphoned from the top to the second, and af- 
terwards to the third steamer, and thus make room for more bulky 
weakly liquor ; from the dissolving lead when a pellicle appears 
on its surface, it should be syphoned off into leads, and left for a 
couple or three days (according to the season) to crystallize. These 
leads need not be of heavier metal than four pounds, and made 
four feet long, two feet wide, and nine inches deep, but their form 
