314 



EVAPORATION 



Tho operation of this apparatus is as follows : aftor opening tho cocks c, /, g, and 

 before admitting tho cold water into tho condenser E, tho cock of the pipe k is opened, 

 in order that by injecting steam it may expel tho included air ; after which the cocks 

 k and g are to be shut. The water must now bo introduced into tho condenser, and 

 tho cock b opened, whereon the liquid to be evaporated rises from the charging back, 

 through tho tube 6, and replenishes tho vacuum pan to tho proper height, as shown l.y 

 tho register glass tube H. Whenever the desired evaporation or concentration i.s 

 effected, the cock c must be closed, the pipe k opened, so as to iill tho pan with steam, 

 and then tho efflux cock a is opened to discharge tho residuary liquor. By shutting 

 the cocks a and Je, and opening tho cock 6, tho pan will charge itself afresh with liquor, 

 ami tho operation will bo begun anew, aftor b has been shut and c opened. 



The contents of tho close water cistern F, may bo drawn off during each operation. 

 For this purpose, the cock /must first be shut, the cold water is to be then run out of 

 tho condenser o, and k and g arc to bo opened. Tho steam entering by k makes the 

 water flow, but whenever the steam itself issues from the cock g, this orifice must be 

 immediately shut, tho cock /opened, and tho cold water again introduced, where- 

 upon the condensed water that had meanwhile collected in the under part of tho 

 refrigerator flows off into the condenser vessel F. Since some air always enters with 

 tho liquor sucked into the pan, it must bo removed at the time of drawing off the 

 water from tho two condensers, by driving steam through the apparatus. This 

 necessity will bo less urgont if the liquor bo made to boil before being introduced into 

 tho vacuum pan. 



Such an apparatus may bo modified in size and arrangement to suit the peculiar 

 object in view, when it will be perfectly adapted for the concentration of extracts of 

 every kind, as well as saline solutions containing vegetable acids or alkalis. The 

 interior vessel A B should be made of tinned or plated copper. For an account of 

 Howard's vacuum pan, made upon the same principle, see SUSAR. 



When a boiler is set over a fire, its bottom should not be placed too near the grate, 

 lest it refrigerate the flame, and prevent that vivid combustion of tho fuel essential to 

 the maximum production of heat by its means. The evil influence of leaving too 

 little room between the grate and the copper may be illustrated by a very simple 

 experiment. If a small copper or porcelain capsule containing water be hold over 

 the flame of a candle a little way above its apex, the flame will suffer no abatement of 

 brightness or size, but will continue to keep the water briskly boiling. If the capsule 

 be now lowered into the middle of the flame, this will immediately lose its brightness, 

 becoming dull and smoky, covering the bottom of the capsule with soot ; and, owing 

 to the imperfect combustion, though the water is now surrounded by the flame, its 

 ebullition will cease. 



Fig. 839 is a section of two evaporating coppers en suite, so mounted as to favour 

 the full combustion of the fuel. A is the hearth, in which wood or coal may bo 



burned. For coal, the grate should be set higher and be somewhat smaller, a i> the 

 door for feeding tho fire ; d, an arch of fire-bricks over tho heart li ; r, a grate thruuh 

 which tho ashes fall into tho pit beneath, capable of being closed in front to any 

 extent by a sliding door b. n and c are two coppers encased in brickwork : /the 

 flue. At the end of the hearth near m, where tho lire plays first upon tho copper, 

 tho solo is made somewhat lower and wider, to promote tho spreading of the flame 

 under the vessel. The second copper c, receives tho benefit of tho wasto heat; it 

 may be placed upon a higher level, BO as to discharge its concentrated liquor by a 

 stop-cock or siphon into the first. 



