EVAPORATION 



315 



810 



Fig. 840 represents a pan for evaporating liquids, which are apt, during cwioontra- 

 tion, to let fall crystals or other sediment. 



These would be injured either by the fire playing upon the bottom of the pan, or, by 

 adhesion to it, they would allow the metal to get red hot, and in that state run every 

 risk of being burnt or rent on the sudden intrusion of a little liquor through the 

 incrustation. When largo coppers have their bottoms planted in loam, so that the 

 flame circulates in flues round their sides, they are said to be cold-set. 



A is a pear-shaped pan, charged with the liquid to be evaporated ; it is furnished 

 with a dome cover, in which there is an opening with a flange/, for attaching a tube, 

 to conduct the steam wherever it may be required, a is the fire-place ; b, the ash-pit. 

 The conical part terminates below in the 

 tube ff, furnished with a stop-cock at its 

 nozzle h. Through the tube c d </, furnished 

 above and below with the stop-cocks c and 

 c', the liquid is run from the charging back 

 or reservoir. During the operation, the 

 upper cock c is kept partially open, to re- 

 place the fluid as it evaporates ; but the 

 under cock c j is shut. The flame from the 

 fireplace plays round the kettle in the space 

 e, and the smoke escapes downwards through 

 the flue i into the chimney. The lower 

 cylindrical part g remains thus compara- 

 tively cool, and collects the crystalline or 

 other solid matter. After some time, the 

 under stop-cock c f , upon the supply-pipe, is 

 to be opened to admit some of the cold liquor 

 into the cylindrical neck. That cock being 

 again shut, the sediment settled, and the 

 large stop-cock (a horizontal side valve would 

 be preferable) h opened, the crystals are 

 suffered to descend into the subjacent receiver ; after which the stop-cock h is shut, 

 and the operation is continued. A construction upon this principle is well adapted 

 for heating dyeing coppers, in which the sediment should not be disturbed, or exposed 

 to the action of the fire. The fireplace should be built as for the brewing copper. 



Fig. 841 represents an oblong evaporating pan, in Avhich the flame, after beating 



841 



along its bottom, turns up at its further end, plays back along its surface, and passes 

 off into the chimney. A is a rectangular vessel, from 10 to 15 feet long, 4 to 6 feet 

 broad, and 1 or H foot deep. The fire-bricks j upon which the pan rests, are so ar- 

 ranged as to distribute the flame equally along its bottom. 



Leidenfrost in 1756 (' Annalesde Chimie') observed some remarkable facts connected 

 with evaporation, which have since received some striking illustration from the 

 experiments of M. Boutigny. 



When water is thrown on a plate heated considerably above the boiling point of 

 water, the liquid assumes a spheroidal form, and this condition has hence received the 

 name of the spheroidal state. This water rolls about like melted crystal without 

 any signs of ebullition, and it is dissipated but very slowly. The explanation usually 

 given is as follows : ' The cause of the phenomena appears to be this : water exhibits 

 an attraction for the surface of almost all solids, and wets them ; fluid mercury exhibits 

 the opposite property, or repulsion for most surfaces. The attraction of water for 

 surfaces brings it into the closest contact with them, and greatly promotes the com- 

 munication of heat by a heated vessel to the water contained in it. But heat appears 

 to develope a repulsive power in bodies, and it is probable that, above a peculiar tem- 

 perature, the heated metal no longer possesses this attraction for water. The water 

 not being attracted to the surface of the hot metal, and induced to spread over it, is 

 not rapidly heated, and therefore boils off slowly.' Grraham. 



