1824.] Mr. Herapath on the Theory of Evaporation. 349 



Article V. 



Addition to Mr. Herapath' 's Theory of Evaporation in the Annals 

 for November, 1821. By John Herapath, Esq. 



(To the Editor of the Annals of Philosophy.) 



DEAR SIR, Cranford, April 12, 1824, 



In Prop. 14 of ray theory of evaporation, it is said that the 

 absolute evaporation is not affected by the pressure of any super- 

 incumbent air. This is to be understood of the decomposition 

 at the surface of the evaporating body, and not of that escape or 

 dispersion of vapour which constitutes apparent evaporation. 

 Against the indiscriminate confusion of these things hints are 

 given, I believe, in more than one place in my theory ; but my 

 attention having been recalled to the subject by the kind inqui- 

 ries of the Rev. E. W.M.Rice, some views respecting the effects 

 of pressure by airs have occurred which I intend here to notice 

 in order to prevent misconception on certain points of my former 



theory. . 



If an evaporating surface were placed in an indefinite vacuum, 

 it is plain that little, or perhaps none, of the emitted vapour 

 would be recondensed on the body, but would expand into 

 space. But were any body as an air, for instance, to be con- 

 fined over the evaporating body, the particles of this air would 

 of course come in contact with many particles of the ascend- 

 ing vapour, and striking them in all directions some would 

 necessarily be beaten back on the evaporating body. Of these 

 a considerable portion would most probably be recondensed. 

 But whatever be the number recondensed, they will evidently be 

 as the number beaten back to the surface, and they as the num- 

 ber struck by the particles of the air. Again the number struck 

 in a given time must be as the number of particles of the air 

 which in the same given time would pass through or strike a 

 given space. Now this number multiplied by the momentum of 

 a single particle, that is by the temperature of the air, is equal to 

 the elastic force of the medium. The number of recondensed 

 particles, therefore, or the momentary effect of a superincumbent 

 air on the incremental evaporation is as the elasticity of the air 

 directly, and its temperature inversely. And as this is demon- 

 strated of no particular air, it is true of any air. 



Hence, therefore, if two evaporating bodies of the same kind be 

 placed in any tivo airiform una) ' traded media, respectively of the 

 temperatures of their contained evaporating bodies, the momentary 

 effects of the media un the evaporations will be in a ratio com- 

 pounded of the elasticities directly, the incremental evaporations 

 directly, and the temperatures inversely. 



And if the temperatures are equal, the momentary ejects are 



