108 THE CHINESE SUGAR CANE. 
it is required to check the froth in the pan, serving also 
to admit air when the sugar is ready to be discharged; 
J is the proof-stick, by which a small quantity of sugar 
may be withdrawn from the pan at pleasure, so that the 
operator may know, from time to time, how the charge is 
progressing. 
The method of operating with the vacuum pan to best 
advantage on a sugar estate, is to boil the sugar as I 
have before advised in the old train, till it reaches a 
density of 30° or 82° Beaumé, after which, introduce 
and boil in vacuo to the crystallizing point. It 
would be too tedious, as well as too expensive, to boil 
down raw cane juice in this way. Rellieux, however, 
by means of three closed pans, operates on the raw juice 
in this way; with what economy I cannot say, never 
having examined the process thoroughly. His train is 
very costly, of course. 
Unlike boiling in the open air, owing to its low tem- 
perature, granulation, to a considerable extent, takes 
place in the vacuum pan, if the operation be properly 
conducted, and the vacuum well preserved, to do which, 
hike everything else, requires practice in the operator. 
The sugar, when discharged, instead of resembling thick 
syrup, as it would if boiled in the open pan, resembles 
thin mortar, being full of grain, and requiring but a 
slight decrease of temperature to become solid. 
THE FRENCH BASCULE PAN. 
At figure 17, I give a representation of the old French 
Bascule pan, which was superseded by Howard’s process. 
