52 



VAPORIZATION. 



stationary while the steam generated 

 was allowed to escape. The steam 

 in the upper part of the globe be- 

 comes denser, more and more steam 

 being produced, and forces the mer- 

 cury to ascend in the gauge tube, 

 / t to a height proportional to the 

 elastic force of the steam. The 

 height of the mercurial column may 

 be taken to express the elastic force 

 or pressure of the steam produced at 

 any particular temperature above 

 212. The weight of the atmosphere 

 itself is equivalent to a column of 

 mercury of 30 inches, and this pres- 

 sure has been overcome by the steam 

 at 212o, before it began to act upon 

 the mercurial gauge. For every 

 thirty inches that the mercury is 

 forced up in the gauge tube by the 

 steam, it is said to have the pressure 

 or elastic force of another atmo- 

 sphere. Thus, when the mercury 

 in the tube stands at thirty inches, 

 the steam is said to be of two 

 atmospheres ; at 45 inches,, of two 

 and a half atmospheres ; at 60 

 inches, of three atmospheres, and so on. 



Experiments have been made on the elastic force of steam 

 by Professor Robinson, Mr. Southern, Mr. Watt, Dr. Ure, and 

 others ; but all preceding results have been superseded by those 

 of a commission of the French Academy, appointed by the 

 French government to investigate the subject, from its import- 

 ance in connexion with the steam engine. Their results, which 

 are expressed in the following table, were obtained by experi- 

 ment, up to a pressure of 25 atmospheres. The higher pres- 

 sures were calculated by extending the progression observed at 

 lower temperatures. 



