158 SCIENCE OF COMMON THINGS. 



Influence of atmospheric pressure on the boiling point. How air is heated. 



less than usual, then the boiling point of water and all 

 other liquids will be lower than usual ; if the pressure 

 increases, and the barometer rises, the temperature of 

 ebullition will be proportionably increased. 



1OSS If the atmospheric pressure be entirely removed, or if water be 

 made to boil in a vacuum, at what temperature will ebullition commence ? 



At a point 140 lower than in the open air. 



1053 To what temperature can water exposed to the air be heated under 

 ordinary circumstances ? 



To about 212 Fahrenheit ; at this temperature water 

 passes into steam or vapor. 



1054 Can water be heated beyond 212 ? 



Yes ; if subjected to sufficient pressure, it can be heated 

 to any extent without boiling. There is no limit to the 

 degree to which water may be heated, provided the 

 vessel is strong enough to confine the vapor ; but the 

 expansive force of steam is so enormous under these 

 circumstances, as to overcome the greatest resistance , 

 which has ever been exerted upon it. 



1055 Why does soup keep hot longer than boiling water f { 



Because the grease and other ingredients floating irf 

 the soup retain the heat longer than the particles of 

 water, and, at the same time, by their viscidity or tena- 

 city, prevent the circulation of the heated particles. 



lOSe How is air heated? 



By " connective currents" 



lOST* Explain what is meant by " convective currents" 



When a portion of air is heated, it rises upwards in 

 a current, carrying the heat with it ; other colder air 

 succeeds, and (being heated in a similar w T ay) ascends 

 also : these are called " convective currents." 



" Convective currents" so called from the Latin words cumvectus (carried 

 with), because the heat is "carried with" the current. 



1058 Is air heated by the rays of the sun f 



"N6 ; air is not heated (to any very great extent) Inj the 

 action of the surfs rays passing through it. 



1059 Why, then, is the air hotter on a sunny day than on a cloudy 

 one? 



