ON SUPERHEATED STEAM. 



193 



Fig. 24. 



comes sub-saturated steam, no matter what the 

 pressure, for superheated or sub-saturated steam 

 is such when it will take up 

 more water. The formation 

 of sub-saturated steam may- 

 be formed, by illustration, 

 with the next diagram of a 

 larger vessel (Fig. 24). 



Suppose half the space to 

 be filled with 1,700 measures 

 of steam, and then allowed 

 to expand into the whole 

 space, or 3,400 measures of 

 steam, it would have only half its elasticity ; but 

 it would not then be saturated steam, but sub- 

 saturated steam, and capable of taking up its own 

 weight of additional water. In other words, it 

 would be superheated steam, having a temper- 

 ature of 212°, but only a pressure of saturated 

 steam at 180°. It would be a dry or drying 

 steam, in which water would evaporate or wet 

 things dry; but 180° would be its dew-point, 

 below which it would condense partly into water, 

 and would become saturated steam. The morn- 

 ing dew or fog is saturated steam, which be- 

 comes sub- saturated with the rising sun, and 

 bodies wet will rapidly dry. Nature is always 

 generating steam and sub- saturating it, and vice 

 versa. 



We now come to the explanation of ''the 







