1821. ] Causes of Calorific Capacity, Latent Heat, Sfc. 441 



decomposition at the surface, and is, therefore, influenced by 

 temperature only, and the other from an internal decomposition 

 which subjects it to the influence of pressure as well as tempera- 

 ture. From even a first view of phenomena at this point, one 

 would infer that the temperatures of tension and ebullition, or 

 the temperature of a fluid at the proper tension of its vapour, and 

 the temperature of its ebullition under an equal pressure, are 

 different ; the latter being higher than the former. Notwithstand- 

 ing the obviousness of this difference, philosophers have never, 

 to the best of my knowledge, discovered it; but have confounded 

 the two phaenomena together. To set the difference in a clear 

 light, let us descend to the temperature of ebullition in vacuo. 

 By Mr. Robison's experiments, it would seem that all fluids boil 

 at about 124° of Fahrenheit less in vacuo than under an atmo- 

 sphere of 30 inches. Water, therefore, will boil at about 88° 

 Fahr. in a vacuum. Now, by Mr. Dalton's experiments, the 

 tension at 88° is 1*28 inches ; but because an increase of pres- 

 sure retards the temperature of ebullition, if we put on the surface 

 of the water an atmospheric pressure of l^- inch, we must 

 evidently increase the temperature to make it boil ; and as it is 

 about the incipient state of ebullition we must probably increase 

 it many degrees. Therefore the temperature at which water 

 boils under a pressure of 1*28 in. is higher than the temperature 

 corresponding to an equal tension. Let us merely, for the sake 

 of continuing the argument, conceive, that with a pressure of 

 1-28, water boils at 100° Fahr. At this temperature the tension 

 is 1*86, or -38 above what it was before. Consequently, if we 

 increase the pressure on the water by -38 so as to make it 1*86, 

 we shall increase its temperature of ebullition, and make it 

 higher than 100°; but at the tension 1*86, the temperature is 

 100° ; hence, therefore, the temperature of ebullition is still 

 above the temperature of tension. By carrying on the argument 

 in this way, it will appear that the temperature of ebullition is 

 always higher than the temperature of tension ; though it is 

 manifest that as the tensions increase, the two temperatures will 

 continually approximate, until at very high tensions no sensible 

 difference can be discovered. 



The following table, computed from the experiments of Mr 

 Dalton and De Luc, strengthens this inference. De Luc, I 

 believe, deduced his boiling point from a pressure of 29*8 in. and 

 Mr. Dalton from a pressure of 30 inches. The one temperature, 

 212° of ebullition, corresponds to about 21L66 of the other; I 

 have, therefore, increased Mr. Dalton's temperatures by •34°, 

 •34°, -33°, -33°, -33°, -31°, -31°, respectively, to make them cor- 

 respond to a like pressure of ebullition. 



