$ 302 



EBULLITION. 



The close coincidence of these early observations of Dr. Black with the re- 

 sults of more recent experiments is worthy of notice. The following are the 

 results of observations made by five distinguished philosophers to ascertain the 

 quantity of heat rendered latent by water in the process of vaporization at 2 12 : 

 Watt, 950 ; Southern, 945 ; Lavoisier, 1,000; Rumford, 1 ,004 8 ; Des- 

 pretz, 955 8. 



The average of all these is about 980 ; so that the round number of 1,000 

 may be taken as a close approximation to the latent heat of steam raised from 

 water at the temperature of 212. 



In order to derive all the knowledge from these experiments which they are 

 capable of imparting, it will be necessary to examine very carefully how water 

 comports itself under a variety of circumstances. 



If water be boiled in an open vessel, with a thermometer immersed, on dif- 

 ferent days, it-will be observed that the fixed temperature which it assumes in 

 boiling will be subject to a variation within certain small limits. Thus, at one 

 time it will be found to boil at the temperature of 210 ; while, at others, the 

 thermometer immersed in it will rise to 213; and, on different occasions, it 

 will fix itself at different points within these limits. It will also be found, if 

 the same experiment be performed at the same time in distant places, that the 

 boiling points will be subject to a like variation. Now, it is natural to inquire 

 what cause produces this variation ; and we shall be led to the discovery of 

 the cause, by examining what other physical effects undergo a simultaneous 

 change. 



If we observe the height of a barometer at the time of making each experi- 

 ment, we shall find a very remarkable correspondence between it and the boil- 

 ing temperature. Invariably, whenever the barometer stands at the same height, 

 the boiling temperature will be the same. Thus, if the barometer stand at 

 thirty inches, the boiling temperature will be 212. If the barometer fall to 

 twenty-nine and a half inches, the thermometer stands at a small fraction above 

 211. If the barometer rise to thirty and a half inches, the boiling temperature 

 rises to nearly 213. The variation in the boiling temperature is, then, ac- 

 companied by a variation in the pressure of the atmosphere indicated by the 

 barometer ; and it is constantly found that the boiling point will remain un- 

 changed so long as the atmospheric pressure remains unchanged, and that every 

 increase in the one causes a corresponding increase in the other. 



From these facts it must be inferred that the pressure excited on the surface 

 of the water has a tendency to resist its ebullition, and to make it necessary, 

 before it can boil, that it should receive a higher temperature ; and, on the con- 

 trary, that every diminution of pressure on the surface of the water will give 

 an increased facility to the process of ebullition, or will cause that process to 

 take place at a lower temperature. , As these facts are of the utmost impor- 

 tance in the theory of heat, it may be useful to verify them by direct experi- 

 ment. 



If the variable pressure excited on the surface of the water by the atmo- 

 sphere be the cause of the change in the boiling temperature, it must happen 

 that any change of pressure produced by artificial means on the surface of the 

 water must likewise change the boiling point, according to the same law. 

 Thus, if a pressure considerably greater than the atmospheric pressure be ex- 

 cited on a liquid, the boiling point may be expected to rise considerably above 

 212; and, on the other hand, if the surface of the water be relieved from the 

 pressure of the atmosphere, and be submitted to a considerably diminished 

 pressure, the water would boil below 212. 



Let B, fig. 3, be a strong spherical vessel of brass, supported on a stand S, 

 under which is placed a large spirit lamp L, or other means of heating it. In 



