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former by boiling, at the ordinary pressure of one atmosphere. If, 

 however, half this weight be removed the steam would occupy double 

 the space. Tiieretoie we say that steam is elastic. But it is not so to 

 an unlimited exient ; for if, instead of diminishing, we were to increase 

 the pressure a large portion of the steam would be converted into water. 

 That is to say, the tension of steam at ioo°C. or its power to withstand 

 pressure, is equal to one atmosphere (the weight of a column of air 

 from the sea-level to the limit of the ataios[)here, equivalent to the 

 weight of a column of mercury of the same diameter 760 mm. high). 

 Steam heated to a higher temperature (as can be done in the boilers 

 of steam engines) can resist a greater force before being converted into 

 water. It is, therefore, able to do some work in addition to resisting 

 the atmosphere. If cooler than ioo°C. its tension is less than that 

 necessary to resist the atmosphere ; and, therefore, being unable to 

 entirely resist it, the steam must be mixed with the air (in proportion 

 depending on the tem])erature) if it is to remain uncondensed. The 

 cooler it is, the greater the proportion of air mixed with it must be; or, 

 since the temperature of the steam and the air are the same, we 

 may say the cooler the air the less aqueous vapour it is able to hold. 



When air is completely saturated with vapour, it is said to be at its 

 deiv poiJit. If subsequently cooled, a portion of the vapour will separate; 

 if heated, it can absorb stiil more. This we find frequently illustrated 

 in nature. A glass of cold water brought into a warm room frequently 

 condenses a film of water on its surface. During cold weather dew is 

 often deposited from the atmosphere of the warmer room on the 

 windows. Clouds and fogs, which consist of minute drops of water too 

 small to fall to the ground, are produced by a warm current of air laden 

 with moisture coming in contact with a colder one, lowering its temper- 

 ature below the dew point. That the clouds surrounding the ptaks of 

 mountains appear to remain there permanently, notwithstanding that a 

 slight wind may be blowing, is due to the cold atmosphere produced 

 by the ice, snow, or glaciers being confined to narrow limits. The warm 

 air striking these produces a cloud which disappears when the warmer 

 region is again reached ; for the drops of water being once more 

 evaporated become invisible like aqueous vapour always is. 



The minute drops of water in the clouds, if gathered together into 



