I 



ON SUPERHEATED STEAM. 189 



limits of my paper, the nature and working of 

 high-pressure steam-boilers, we will enter upon 

 the consideration of superheated steam, or, more 

 properly speaking, sub-saturated steam, and its 

 generation. 



Now, the difference between the nature of 

 saturated and sub-saturated steam is fully ex- 

 pressed by the words used; but words, I am 

 aware, do not always convey a knowledge of 

 what is, or may be, correctly expressed, and 

 therefore I shall endeavour to explain what sub- 

 saturated steam is. Steam is saturated which is 

 in contact with water, and cannot be anything 

 else, no matter what heat is applied and what 

 pressure is attained, as long as it is in contact 

 with water. This step is, I trust, perfectly under- 

 standable. 



Steam which is not in contact with water, and 

 which is heated ever so little or ever so much, is 

 superheated or sub- saturated steam. This differ- 

 ence is clear between the two kinds of steam, and 

 cannot tax the memory much to retain it — satu- 

 rated steam is wet steam, sub-saturated is dry 

 steam. 



Having clearly in your minds the many dangers 

 and difficulties to contend with in high-pressure 

 steam-boilers, you will be prepared, I hope, to see 

 and understand that there is an absence of them 

 in the use of superheated steam-chambers. 



