242 WELLS'S NATURAL PHILOSOPHY. 



„ x|^ 559. As we ascend into the atmosphere the pressure is di- 



temperature at minished, because there is less of it above us ; it thereibro 

 boils^'be "iised Allows, that water at different heights in the atmosphere will 

 for determin- boil at different temperatures, and it has been found by ob- 

 ng e evationg servation, that an elevation of 550 feet above the level of tha 

 sea causes a difference of one degree in its boiliBg point. Hence the boiling 

 point of water becomes an indication of the height of any station above the 

 sea-level, or in other words, an indication of the atmospheric pressure ; and 

 thus by means of a kettle of boiling water and a thermometer, the height of 

 the summit of any mountain may be ascertained with a great degree of ac- 

 curacy. If the water boils at 211° by the thermometer, the height of the 

 place is 550 feet ; if at 210°, the height is 1100 feet, and so on, it being only 

 necessary to multiply 550 by the number of degrees on the thermometer 

 between the actual boiling" point and 212°, to ascertain the elevation. In the 

 city of Quito, in South America, water boils at 194° 2" F. ; its height above 

 the sea-level is, therefore, 9,541 feet. 



As we descend into mines, the pressure of the atmosphere is increased, there 

 being more of it above us than at the surface of tlie earth. "Water, therefore, 

 must be heated to a higher temperature before it will boU, and it has been 

 found that a descent of 550 feet, as before, makes a difference of one degree. 

 560. In a hke manner, if by artificial means we increase or 

 toiling point diminish the pressure of the atmosphere on the surface of a 

 chan^e'd'^^rti- ^^1^^'^' ^'^ change its boihng point. If water be heated in a 

 ficiaUy? vacuum, ebullition will commence at a point 140° lower than 



in the open air. If a vessel of ether be placed under the re- 

 ceiver of an air-pump, and the atmospheric pressure removed from its surface, 

 the vapor rises so abundantly that ebulhtion is produced without any in- 

 crease of temperature. 



How is su-'ar Several beautiful applications in the arts have been made 



boiled in the of the principle that liquids boil at a lower temperature when 



fining^ °^ ^^' ^^^®^ ^^""^ *^^° pressure of the atmosphere than m the open 

 air. 



In the refining of sugar, if the syrup is boiled in the open air, the tempera- 

 ture of the boiling point is so high that portions of the sugar become decom- 

 posed by the excess of heat, and lost or injured ; the syrup is therefore boiled 

 in close vessels from which the air has been previously exhausted, and in this 

 way the water of the syrup may be evaporated at a temperature so low as to 

 prevent all injury from heat. 



For cooking, this application could not be carried out. The water might, 

 Indeed, be made to boil at a temperature much less than 212°, but owing to 

 its diminished heat would not produce the desired effect. 



whatisdista- 561. Distillation is a process by which one 



ation? hody is separated from another by means of 



heat, in cases where one of the bodies assumes the form 



of vapor at a lower temperature than the other ; this first 



