CHAPTEE XYI. 



THE EVAPORATION OF THE JUICE TO SYRUP. 



By evaporation is meant the passage of a body from the liquid to the 

 gaseous state. The term evaporation is usually restricted to that change 

 which takes place continuously at the surface of a liquid, and ebullition or 

 boiling to that which takes place throughout the entire mass. 



Evaporation takes place continuously at the surface of all exposed liquids. 

 The molecules or particles of matter, of which all bodies are supposed to be 

 composed, have in a liquid but little attraction for each other, and tend 

 constantly to fly off into space. In an enclosed liquid a quantity of vapour 

 will be formed above the surface. The molecules of the liquid, in the form of 

 vapour, are in constant movement, and will in their paths to and fro impinge 

 on the surface of the liquid, which is at the same time constantly giving off 

 vapour; eventually a point is reached when as many molecules leave the 

 liquid as enter, so that a system in equilibrium obtains. When this state is 

 reached the space above the liquid is said to be saturated, and no more 

 evaporation (apparently) takes place. 



The vapour given off by a liquid exerts a certain pressure which increases 

 with the temperature ; this pressure is known as the vapour tension. When 

 the vapour tension of the liquid is equal to the pressure of the surrounding 

 atmosphere, bubbles of vapour form in all parts of the liquid, and the latter is 

 said to boil ; the boiling point of a liquid is then not a fixed figure, but is 

 dependent on the pressure under which it occurs. The normal pressure of the 

 atmosphere at sea level is equal to that exercised by a column of mercury 

 29-92 inches or -760 metre high; this is equal to a pressure of 14707 Ibs. per 

 square inch or to 1-034 kilogram per square cm. At this pressure water boils 

 at a temperature of 212F. or 100C. ; if the pressure be decreased the 

 boiling-point of water of any liquid falls, and it is due to this phenomenon that 

 multiple effect evaporating apparatuses can be constructed, in which 1 Ib. 

 of steam may evaporate in practice up to 5 or 6 Ibs. of water. A table is 

 added at the end of this chapter, giving data connecting the pressure and 

 temperature of steam. The temperature at which a liquid boils is also 

 influenced by the presence of bodies in solution ; a table of the boiling- 

 points of sugar solutions under atmospheric pressure will be found elsewhere. 



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