VACU1~M PAXS. 341 



pan must be repeatedly washed and rinsed with water, to remove ever\- 

 portion of the acid. 



A method which has been found effective with the open pan, has 

 been to wipe the pan quite dry, and before using it to burn a small 

 armful of straw or shavings under the pan, the sudden expansion of 

 the iron by the momentary- blaze cracking the scale loose, so that it 

 may be swept out of the pan with a broom. Of course, care must be 

 taken to avoid unsoldering of the pan in this operation, of which there 

 is no danger if the blaze is only of short duration. It is verv interest- 

 ing to observe, that a similar scale is often found in the pans when 

 maple sap is evaporated ; but this scale has been found to be the lime 

 salt of malic, another organic acid. 



VACUU^I PAJN'S. 



The principle upon which the vacuum pan is based, is the fact that 

 the boiling point of water, syrup, or any liquid, is in part, dependent 

 upon the pressure of the atmosphere, and that the temperature at 

 which a liquid boils, is higher or lower, according as the atmospheric 

 pressure is increased or diminished. For example, the boiling point 

 of water at the sea level, is 212° F. (100° C), and the pressure of the 

 atmosphere is equal to the pressure of a column of mercury 30 inches 

 (776 mm.) high ; but it is found that, at the summit of Mount Blanc, 

 which is 15,560 feet above the level of the sea, water boils at 185°. 8F. 

 (85°. 6 C), and that the pressure of the atmosphere at the summit, is 

 only equal to a column of mercury 17.363 inches high (449 mm.) 



It appears, then, an elevation of ab nit 600 feet diminishes the boil- 

 ing point 1°F., and this is demonstrated as due to the fact, that the 

 pressure of the atmosphere is diminished by leaving a portion of the 

 atmosphere below us in the ascent. Conversely, if we descend a mine, 

 we fiud the boiling point increases, owing to increase of pressure, as 

 indicated by the barometer. 



If, now, by artificial means, the pressure of the atmosphere is in 

 part or wholly removed from a liquid, we find the boiling point is pro- 

 portionably diminished ; and in fact, the boiling point of water has been 

 reduced in a vacuum approximately perfect to 70°F. (21°. IC). 



The vacuum pan, which was invented by Howard in 1813, and 

 which has come into such general use in the evaporation of liquids, 

 works by the principle above illustrated. Its importance in su^ar 

 making is due to the fact, that, by the avoidance of high temperatures, 

 there is no danger of burning the sugar, as is frequent with open pans; 

 and the inversion of the susrar is reduced to a minimum. 



