304 



EBULLITION. 



the tube A being open at the top, the atmosphere presses on the surface of the 

 mercury in it. The thermometer T will be observed gradually to rise until it 

 attains the temperature of 212 ; but it will not stop there, as it would do if 

 immersed in water boiled in an open vessel. It will, on the other hand, con- 

 tinue to rise ; and when the column of mercury in A has attained the height 

 of 30 inches, the thermometer T will have risen to 250, being 18 above the 

 ordinary boiling point. 



During the whole of this process, the surface of the water being submitted 

 to a constantly increasing pressure, its ebullition is prevented, and it continues 

 to receive heat without boiling. That it is the increased pressure which re- 

 sists its ebullition, and causes it to receive a temperature above 212, may be 

 easily shown. Let the- stopcock C be opened : immediately the steam in B, 

 having a pressure considerably greater than that of the atmosphere, will rush 

 out, and will continue to issue from C, until its pressure is balanced by the 

 atmosphere. At the same time the column of mercury in A will be observed 

 rapidly to fall, and to sink below the orifice by which it is inserted in the ves- 

 sel B. The thermometer T also falls until it attains the temperature of 212. 

 At that point, however, it remains stationary ; and the water will now be dis- 

 tinctly heard to be in a state of rapid ebullition. If the stopcock C be once 

 more closed, the thermometer will begin to rise, and the column of mercury 

 ascending in A will be again visible. 



If, instead of a stopcock being at C, the aperture were made to communicate 

 with a valve, like the safety-valve of a steam-engine, loaded with a certain 

 weight, say at the rate of fifteen pounds on the square inch, then the thermom- 

 eter T, and the mercury in the tube A, would not rise indefinitely as before. 

 The thermometer would continue to rise till it attained the temperature of 250, 

 and the mercury in the tube A would rise to the height of 30 inches. At this 

 limit the resistance of the valve would be balanced by the pressure of the 

 steam ; and as fast as the water would have a tendency to produce steam of a 

 higher pressure, the valve would be raised, and the steam suffered to escape ; 

 the thermometer T and the column of mercury in A remaining stationary du- 

 ring this process. If the valve were loaded more heavily, the phenomena 

 would be the same, only that the mercury in T and A would become stationary 

 at certain heights. But, on the other hand, if the valve were loaded at a less 

 pressure than fifteen pounds on the square inch, then the mercury in the two 

 tubes would become stationary at lower points. 



These experiments show that every increase of pressure above the ordinary 

 pressure of the atmosphere causes an increase in the temperatnre at which 

 water boils. We- shall now inquire whether a diminution of pressure will pro- 

 duce a corresponding effect on the boiling point. 



This may be easily accomplished by the aid of an air-pump. Let water at 

 the temperature of 200 be placed in a glass vessel under the receiver of an 

 air-pump, and let the air be gradually withdrawn. After a few strokes of the 

 pump the water will boil ; and if the mercurial gauge of the pump be observed, 

 it will be found that its altitude will be about twenty-three and a half inches. 

 Thus the pressure to which the water is submitted has been reduced from the 

 ordinary pressure of the atmosphere expressed by the column of thirty inches 

 of mercury to a diminished pressure expressed by twenty-three and a half 

 inches ; and we find that the temperature at which the water boils has been 

 lowered from 212 to 200. Let the same experiment be repeated with 

 water at the temperature of 180, and it will be found that a further rarefac- 

 tion of the air is necessary, but the water will at length boil. If the gauge of 

 the pump be now observed, it will be found to stand at about fifteen inches, 

 showing that at the temperature of 180 water will boil under half the ordinary 



