EVAPORATION, HOW CONDUCTED. 103 
the scum rapidly forward and down into the recesses R R, 
where it lodges, and, as it cools, it becomes densely com- 
pacted and displaces any unclarified juice that may have 
collected there at the commencement of the operation. At 
the same time that the scum is being separated and carried 
forward, an under-current of celd juice is continually pass- 
ing onward in the opposite direction and becoming rapidly 
heated. Ata certain point (A) as it advanees, it is sud- 
denly thrown into ebullition, when it parts company with 
the scum, and impelled onward by the colder current be- 
hind it, it escapes by the gate P into B. The gates P P, 
the openings of which are protected by wire gauze, afford 
at any time a convenient exit also into B, for any elarified 
juice which may be thrown into the bays when they are vot 
packed with scum. Ordinarily, however, these gates are 
not used as outlets for any considerable quantity of the 
juice into B, as there is sometimes danger of unclarified 
juice passing through them. They should be closed at the 
commencement of boiling, and afterward opened only just 
enough to allow the small proportion of boiled juice upon 
which the scum has floated into the bays, to slowly filter 
through them. A return current is thus prevented, and 
the scum is forced back into the head of the bays and re- 
mains there. At. convenient intervals, once every half 
hour or more, this dense mass of scum is lifted out bodily 
by means of a large flat dipper or shovel of such width as 
to slide closely inside the bays. 
A strong heat must be uniformly maintained in the fur- 
nace. Feed the fire with regularity, and do not add so 
much fresh wood at once as at any time to check the boil- 
ing in the front pan. If at any time this should occur, 
close the entrance gate into B, and stop the flow of juice 
from the tank until vigorous boiling is resumed. When 
coal can be had, it may be used in small quantity along 
