372 Mr. Herapath on True Temperature, and the [Nov. 



fluid, the temperature being the same, the megethmerin is 

 always the same, however much the space occupied by the 

 vapour is augmented or diminished, provided there be a sufficient 

 quantity of fluid to supply the vapour required. Or at the same 

 temperature, the elasticity is the same whatever be the space 

 the vapour is made to occupy. 



I have already shown that the higher the temperature of any 

 fluid, the greater is its incremental evaporation, and vice versa. 

 But at the same temperature, the cause being the same, the 

 increments of evaporation must be the same. I have likewise 

 shown that the incremental condensation of any vapour, the 

 temperature being the same, is as its elasticity. If, therefore, a 

 quantity of vapour be confined over its fluid, the excess or defect 

 of the evaporation of the fluid will increase or diminish the 

 quantity of vapour until its condensation be equal to the contem- 

 poraneous evaporation of the fluid, or until the elasticity of the 

 fluid has attained a certain force. When the elasticity has 

 attained this force, if it be attempted to be increased or dimi- 

 nished by forcing the fluid into the same space occupied by the 

 air, or by withdrawing it, a part of the vapour will be condensed, 

 or a part of the fluid evaporated, until the equilibrium of conden- 

 sation and evaporation, and, therefore, the proper elasticity of 

 the vapour, be restored. 



This elasticity, which makes the condensation of the vapour 

 equal to the evaporation of the fluid, I shall henceforward, after 

 the example of Mr. Dalton and M. Biot, call the tension of the 

 vapour. 



Prop. XIV. Theor. XII. 



The temperature being the same, the incremental evaporation 

 of any fluid is the same, however great or however small the 

 aeriform pressure on its surface, or however great or however 

 little the quantity of superincumbent vapour. 



This theorem is to be understood as true only in evaporation 

 strictly so called, that is, a decomposition at the surface, and 

 not that interior decomposition which produces ebullition. 



Because the pressure of an incumbent atmosphere acts by 

 repeated impulses on the surface of the body, and not by a con- 

 stant pressure surrounding and squeezing together the parts of 

 the particles, the action of such an atmosphere, provided its 

 temperature be the same as that of the fluid, however great its 

 compression, will have no influence to accelerate or retard the 

 corpuscular decomposition, or the absolute evaporation of the fluid* 

 And since this is true of superincumbent airs in general, it must be 

 true of superincumbent vapours ; and, therefore, the quantity of 

 superincumbent vapour has no effect on the incremental evapo- 

 ration of the fluid. Consequently the temperature being the 

 same, every fluid evaporates equally fast in a vacuum, and under 

 an atmosphere of any compression ; and equally fast in what is 

 called a damp and in a dry atmosphere. 



