TENSION OF STEAM. 



799 



too great the india- rubber tube may be forced off the glass tube, 

 instead of its being torn or the glass broken by the pressure. 



The tension of steam is measured by the height of 

 a column of mercury which it is capable of supporting. 

 The height of this column is : 



at 



,,10 

 ,,20 

 ,,BO 

 ,,40 

 ,, 50 

 ,,60 

 70 



Millimetres. 



354-6 



525-4 



760-0 



1075-0 



1491-0 



2030-0 



2718-0 



3581-0 



For temperatures above 100 the tension of steam 

 ' is often stated in numbers which express by how many 

 times the tension is greater than the normal pressure 

 of the atmosphere, which is taken as 760 mm ; this 

 normal pressure is thus the unit of comparison, and is 

 usually simply called ' one atmosphere. 7 Compared with 

 this unit the tension of steam is 



at 100 degrees 

 120-6 

 138-9 

 144-0 

 152-2 

 159-6 

 165-4 

 170-8 

 175-8 

 180-3 



1 atmosphere 



2 atmospheres 

 3 



4 

 5 

 6 

 7 

 8 

 9 

 10 



The tension of vapour at any temperature below 100 

 may be proved to be less than the pressure of the 

 atmosphere by the following experiment. A retort, 

 half filled with water, is closed by a cork perforated for 

 the insertion of a glass tube 10 cm long and 5 mm wide. 



