104 



MANUAL FOR SUGAR GROWERS. 



fifteen pounds per square inch. This is measured 

 by means of the barometer, in which the mercury 

 usually stands at a height of about thirty inches. 

 From this it follows that a liquid boils at a higher 

 temperature when the barometer rises, and at a 

 lower temperature when it falls. 



If, by means of an air-pump, the vapour be re- 

 moved from above a liquid boiling in a closed ves- 

 sel, the temperature of the boiling-point will be 

 reduced, according to a well-known law. In the 

 case of water, the following table gives the boiling- 

 point under various pressures : 



Now, from the figures given in connection with 

 the manufacture of muscovado sugar, it will be seen 

 that it is the increased heat, as the syrup becomes 

 concentrated, which leads to such rapid inversion 

 and consequent loss. Hence, by boiling the syrup 

 under reduced pressure we have the means of pre- 

 venting this loss. This, in practice, is carried out by 

 means of the vacuum pan (Fig. 16), which consists 

 of a closed vessel, A, in which the syrup is boiled, the 

 heat being supplied by a series of steam coils, B, at 

 different heights. The steam, as fast as it is formed, 

 is drawn off through the pipe, C, by means of the 



