VACUUM PANS. 343 



and removed by the vacuum pump. Coudensation is effected by a jet 

 of water, similar to the method of condensing engines, or, when water 

 is limited, surface condensation is made use of: the vapor passing 

 through a system of pipes, over which water is allowed to trickle. In 

 the one case, the water used in condensation, as also the condensed 

 vapor, is removed by a steam pump ; in the latter, only the condensed 

 vapor, and such une -ndensed portion as remains, is removed by the 

 pump. At the bottom of the vacuum pan a sliding valve permits the 

 discharge of the contents, when concentration is completed. The 

 semi- syrup is forced into the pan through a pipe connected with the 

 syrup tank, by atmospheric pressure, owing to the vacuum in the pan. 

 A barometer and thermometer are connected with the interior of the 

 pan, by which the extent of the vacuum and the temperature of the 

 contents are indicated at a glance ; also a proof stick, which enables a 

 portion of the contents of the pan to be removed for examination 

 without breaking the vacuum. Eye-glasses in the sides of the pan 

 enable the sugar-boiler to watch and thus control the operations. 



The volume which a gas occupies being inversely as the pressure, 

 it follows that, with a vacuum of 28 inches, a cubic foot of water (7i 

 gallons), which would produce, at the ordinary pressure of the at- 

 mosphere, 1700 cubic feet of steam, would yield, at 28 inches of 

 vacuum, 25,500 cubic feet. It is found that a cubic foot of water 

 will produce 21,500 cubic feet of steam under a pressure of 2 inches 

 of mercury. 



The pipe which is to discharge from the pan the enormous bulk of 

 rarified steam generated, must be adequate, as also the apparatus for 

 condensation. 



