106 THE CHINESE SUGAR CANE. 
top and bottom. It will be perceived that the whole 
force of the steam is applied to small portions or columns 
of liquor at atime; thus an immediate evaporation takes 
place almost as soon as the steam enters the tube cylin- 
der, while all danger of overheating is avoided, by the 
well known tendency of liquids to rise when heated. 
A current sets up through the tubes, the cooler portion 
entering below, heating as it rises, and flowing out at the 
top, until, when the whole mass gets thoroughly heated, 
the juice in ebullition takes the form of a fountain, and 
vaporization goes on with wonderful rapidity. This pan 
is fast coming into favor, and that it will ultimately su- 
persede all other methods of open evaporation by steam, 
is the opinion of planters who have it in use. 
BOILING IN VACUO. 
Besides the preceding, there is yet another method of 
evaporating, the apparatus for which is more complicated 
and expensive than either of the others; this is boiling 
‘in vacuo,” and was invented by Lord Howard, a scien- 
tific Englishman, many years ago. Socomplete was the 
apparatus he adopted, that, with a few minor: additions, 
his pan remains in principle unaltered to this day. 
The principle involved in the vacuum pan is that by 
which watery liquids may be made to vaporize in a 
vacuum at a much lower temperature than in the open 
air. ‘Thus, water boils in the open air at 212°, while in 
vacuo the same takes place at 90°. Advantage is taken 
of this in sugar boiling to extract the water from solutions 
of sugar at a low temperature, and so preserve its color. 
