PRESSURE OF THE AIR. 185 



being, and is also highly necessary to that of 

 every living thing, and to the preservation of 

 natural things in their present condition. To 

 ourselves, it is important in various ways. It 

 prevents that excessive evaporation from the 

 surface of our bodies, which would otherwise 

 exhaust and destroy us. It also determines, in 

 an important manner, the proper density of the 

 volume of air which is requisite for our com- 

 fortable breathing: hence the laborious respi- 

 ration of persons at high elevations. It is im- 

 portant also to us in an economical point of 

 view : it is found, for instance, that if we by 

 any method diminish the pressure on the surface 

 of liquids, as, for example, by exhausting a part 

 of the air from a close vessel, that they will 

 boil at a much lower temperature than when 

 ' the full pressure of the atmosphere is allowed 

 to exert itself upon them. In sugar refineries 

 this is taken advantage of, by boiling down the 

 sugar in pans, out of which the air is pumped 

 by a steam engine; in this way all risk of 

 burning is avoided. Again, if the pressure is 

 increased, the boiling point rises also, and it 

 will be more difficult to bring liquids up to this 

 point. As a curious result of the diminished 

 pressure lowering the boiling point, it may be 

 mentioned, that much inconvenience is occa- 

 sionally felt by travellers on high mountain 

 regions. Darwin relates a humorous anecdote 



