EBULLITION. 



307 



attempted to be formed from the results of these tables, by which the elasticity 

 may in general be deduced from the temperature, and vice versa. 



Another remarkable property which steam enjoys, in common with the air 

 and the gases, is its extreme lightness compared with the ordinary weight of 

 bodies in the liquid and solid forms ; when water is boiled under the medium pres- 

 sure of the atmosphere, the barometer standing at thirty inches, the steam which 

 is produced from it is, bulk for bulk, nearly seventeen hundred times lighter 

 than the water from which it is raised. Thus, a cubic inch of water, when con- 

 verted into steam at 212, will produce about seventeen hundred cubic inches 

 of steam. At a first view it might be supposed that this enormous increase of 

 bulk might proceed from the circumstance of some other body being combined 

 with the water in forming the steam ; but that this is not the case, or, at least, 

 that no ponderable body is so combined with it, may be determined by weigh- 

 ing the steam and the water respectively. These weights will always be 

 found, as already stated, to be equal. This expansion which water undergoes 

 in its transition from the liquid to the vaporous state is subject to great varia- 

 tion, as we shall presently explain, according to the temperature and pressure 

 at which it is raised. 



In the experiment already described, by which the latent heat of steam was 

 determined, the water was supposed to be boiled under the ordinary pressure 

 of the atmosphere. Having seen, however, that water may boil at different 

 temperatures under different pressures, the inquiry presents itself, whether the 

 heat absorbed in vaporization at different temperatures and under different pres- 

 sures, is subject to any variation ? Experiments of the same nature as those 

 already described, instituted upon water in a state of ebullition at different tem- 

 peratures as well below as above 212, have led to the discovery of a very 

 remarkable fact in the theory of vapor. It has been found that the heat ab- 

 sorbed by vaporization is always less, the higher the temperature at which the 

 ebullition takes place ; and less, by the same amount as the temperature of 

 ebullition is increased. Thus, if water boil at 312, the heat absorbed in 

 ebullition will be less by 100 than if it boiled at 212; and again, if water 

 be boiled under a diminished pressure, at 112, the heat absorbed in vaporiza- 

 tion will be 100 more than the heat absorbed by water boiled at 212. It 

 follows, therefore, that the actual consumption of heat in the process of vapori- 

 zation must be the same, whatever be the temperature at which the vaporiza- 

 tion takes place ; for whatever heat is saved in the sensible form is consumed 

 in the latent form, and vice v?.rsa. 



Let us suppose a given weight of water at the temperature of 32 to be ex- 

 posed to any regular source by which heat may be supplied to it. If it be 

 under the ordinary atmospheric pressure, the first 180 of heat which it re- 

 ceives will raise it to the boiling point, and the next 1,000 will convert it into 

 steam. Thus, in addition to the heat which it contains at 32, the steam at 

 212 contains 1,180^ of heat. But if the same water be submitted to a pres- 

 sure equal to half the atmospheric pressure, then the first 148 of heat which 

 it receives will cause it to boil, and the next 1,032 will convert it into vapor. 

 Thus, steam at the temperature of 180 contains a quantity of heat more than 

 the same quantity of water at 32, by 1,032 added to 148, which gives a 

 sum of 1,180. Steam, therefore, raised under the ordinary pressure of the 

 atmosphere at 212, and steam raised under half that pressure at 180, contain 

 the same quantity of heat, with this difference only, that the one has more 

 latent heat, and less sensible heat, than the other. 



From this fact, that the sum of the latent and sensible heats of the vapor of 

 water is constant, it follows that the same quantity of heat is necessary to con- 

 vert a given weight of water into steam, at whatever temperature or under 



