The vacuum-pan was originally heated solely by the admission of steam between the double bottom; but of late years the heat has been also applied to the syrup through several coils of pipe placed within the pan, filled with steam at a temperature many degrees above 212° F., sometimes so high as 250°. By this double application of heat, the evaporating power of a pan has been vastly increased. The latest made pans have a considerably flat bottom, [fig. 1090.]; a spiral pipe, laid close upon it; and between the under hemisphere and the upper one, there is a space a, a, 21⁄2 feet high, to give the syrup room for frothing up without boiling over. The space b, of the bottom receives steam of common pressure, and the spiral tubes, of high pressure. A pan like this is now making for a house in London, which is to work off 16 tons of sugar-loaves daily.
The proof-stick, [fig. 1095.], consists of a cylindrical rod, capable of being screwed air-tight into the pan in an oblique direction downwards. The upper or exterior end is open; the under, which dips into the syrup, is closed, and has on one side a slit a ([figs. 1091], [1092.]), or notch, about 1⁄2 inch wide. In this external tube, there is another shorter tube b, capable of moving round in it, through an arc of 180°. An opening upon the under end e, corresponds with the slit in the outer tube, so that both may be made to coincide, [fig. 1091.] A. A wooden plug d, is put in the interior tube, but so as not to shut it entirely. Upon the upper end there is a projection or pin, which catches in a slit of the inner tube, by which this may be turned round at pleasure. In the lower end of the plug there is a hole e, which can be placed in communication with the lateral openings in both tubes. Hence it is possible, when the plug and the inner tube are brought into the proper position, A, [fig. 1091.], to fill the cavity of the wooden rod with the syrup, and to take it out without allowing any air to enter. In order to facilitate the turning of the inner tube within the outer, there is a groove in the under part, into which a little grease may be introduced.
Whenever a proof has been taken, the wooden plug must be placed in reference to the inner tube, as shown in [fig. 1091.] c, and then be turned into the position A; when the cavity of the plug will again be filled with syrup. c must be now turned back to the former position, whereby all intercourse with the vacuum-pan is cut off; the plug being drawn out a little, and placed out of communication with the inner tube. The plug is then turned into the position B, drawn out, and the proof examined by the fingers.
Table showing the boiling point of syrup, at the corresponding atmospheric pressure within the vacuum-pan:—
| Height of the mercury (inches) in one leg of the syphon, above that in the other— | ||||||||||||
| 0·74 | 0·86 | 1·01 | 1·17 | 1·36 | 1·57 | 1·80 | 2·05 | 2·36 | 2·72 | 3·10 | 3·52 | 4·00. |
| Boiling point, Fahr.— | ||||||||||||
| 115° | 120° | 125° | 130° | 135° | 140° | 145° | 150° | 155° | 160° | 165° | 170° | 175°. |
The large double steam-basin, which receives several successive skippings of the concentrated granulating syrup, serves to heat it from the temperature of 160° or 170°, at which it leaves the vacuum-pan, up to 200° or thereby, before it is filled out into the moulds; for were it introduced in the cooler state, it would not concrete into sufficiently compact loaves.