152 THE ACTION OF HEAT. 
Happily, the problem of rapid evaporation at a low 
temperature has already been solved. The application of 
the principle, that rapid boiling and evaporation will take 
place at a low temperature under diminished atmospheric 
pressure, was a great stride in the progress of sugar manu- 
facture. And now the vacuum pan of Howard, or of 
others who have variously modified it, is used wherever 
skill and large capital are combined in the production of 
sugar from the tropical cane. In the manufacture of sugar 
from sorghum, wherever a steam engine is used to propel 
the crushing mill, a vacuum finishing pan of the simplest 
form would unquestionably be better than any other: the 
steam furnishing the heat to the jacket of the boiler or to 
the coils within it, and the engine driving the air pump by 
which the exhaustion is maintained. By this means the 
syrup, previously boiled to the proper density in the open 
air, is reduced with wonderful rapidity to the proper de- 
gree of concentration, at a temperature not exceeding 170° 
F., the sugar which results after complete crystallization 
being not only somewhat more abundant, but also superior 
in fairness of color and sharpness of grain to that obtained 
by the use of an open-air finishing pan. At the same time 
the molasses is reduced in quantity, and what is more im- 
portant, it is of so much better quality as to furnish, after 
reboiling by the same means, a second crop of crystals but 
little inferior to the first. 
These advantages will no doubt secure the adoption of 
the vacuum finishing pan in all sugar works wherein capital 
is largely invested, but its expensiveness will preclude its 
general use. When a vacuum pan is employed, the syrup 
should pass through the same process in the evaporating 
range as before mentioned, only it should not be retained 
so long in the last two pans of the series, but should pass 
