Sugar Producing Plants. 469 
now very thin still, but perfectly bright and clear, and is 
ready for concentration. This is done in two stages: first, 
it is thickened to a syrup, containing 50 per cent. of sugar, 
in what is known as a double or triple effect. This is a 
peculiar and ingenious apparatus constructed first by a 
Frenchman named Rillieux, and consists of two or three 
cylinders about ten feet in height and six feet in diameter. 
They each contain a series of vertical or horizontal steam 
pipes for boiling the liquor, and communicate with each 
other, so that the vapor from the boiling liquor in the first 
boils the liquor in the second, and that from the second 
boils the liquor in the third. In this way we greatly econo- 
mise the heat. 
There isa further peculiarity about the machine, and that 
is, that to the third cylinder is attached an air pump, which 
sucks all the hot vapor from it as the sugar boils, and draws 
it through a stream of cold water, thus producing a vacuum, 
The object of this is to evaporate the water in the liquor at 
-a low temperature, for, by the well-known law of physics, 
the less the pressure on the surface of a liquid the less heat 
it takes to cause it to boil—that is, to evaporate. We do 
not do this to save fuel, for we have to use more than we 
gain in driving the pump, but we do it to save the sugar, 
for if sugar-liquor is boiled at the pressure of the atmos- 
phere, it becomes partially destroyed by the heat and gets 
quite dark in color. The boiling of liquor in a vacuum is 
the greatest advance made yet in sugar-making, and was 
known long before the principle of the multiple evapo- 
rator. In fact, the vacuum pan, which is the next piece of 
apparatus we have to consider, was long the great centre of 
the sugar factory, and the most difficult and important pro- 
cess was the boiling of sugar, We do not look on the mat- 
ter now with the same awe that our progenitors did, but 
consider it still a most important station, 
The syrup on leaving the evaporator is now quite thick 
and is dark brown in color. It is customary now, in the 
best factories, to boil it up at once in the vacuum pan, but 
many still adhere to an older process, that of bleaching by 
