EBULLITION. 



303 



Pig. 3. 



the top of this vessel are three apertures, in two of which are screwed a ther- 

 mometer T, the bulb of which enters the hollow brass sphere, and a stopcock 

 C, which may be closed or opened at pleasure, to confine the steam, or allow 

 it to escape. In the third aperture, at the top, is screwed a long barometer 

 tube, open at both ends. The lower end of this tube extends nearly to the 

 bottom of the spherical vessel B. In the bottom of this vessel is placed a 

 quantity of mercury, the surface of which rises to some height above the lower 

 end Of the tube A. Over the mercury is poured a quantity of water, so as to 

 half fill the vessel B. Matters being thus arranged, the screws are made tight 

 so as to confine the water, and the lamp is allowed to act on the vessel ; the 

 temperature of the water is raised, and steam is produced, which, being con- 

 fined within the vessel, exerts its pressure on the surface of the water, and 

 resists its ebullition. The pressure of the steam acting on the surface of the 

 water, is communicated to the surface of the mercury, and it forces a portion 

 of the mercury into the tube A, which presently rises above the point where 

 the tube is screwed into the top of the vessel B. As the action of the lamp 

 continues, the thermometer T exhibits a gradually increasing temperature ;' 

 while the column of mercury in A shows the force with which the steam 

 presses on the surface of the water in B, this column being balanced by the 

 pressure of the steam. Thus, the temperature and pressure of the steam at 

 the same moment may always be observed by inspecting the thermometer T 

 and the tube A. When the column in the tube A has risen to the height of 

 30 inches above the level of the mercury in the vessel B, then the pressure of 

 the steam will be equivalent to double the pressure of the atmosphere, because 



