350 Mr. Herapath on the Theory of Evaporation. [May, 



directly as the elasticities and rates of evaporation ; and tvhen the 

 elasticities are equal, these effects are inversely as the temperatures, 

 and directly as the rates of evaporation. Of course in the same 

 fluid at the same temperature, the incremental diminution is as the 

 pressure directly. 



It would be easy to reduce the above laws to a comprehensive 

 integral expression ; but as I know not a single experiment with 

 which it can be compared, I am not disposed to indulge in useless 

 calculations. Besides it appears to me there is another cause 

 Very materially influencing the quantity of apparent evaporation ; 

 and of which I am likewise destitute of any experiments to afford 

 me the least aid towards numerical comparison. It is this. In 

 the above views we have regarded the superincumbent air as 

 totally devoid of gravitation, and affecting the evaporation by 

 its elastic force only. This however seems not to be the whole 

 operating cause. Though by our theory, confirmed by expe- 

 rience, all airs in contact pretty rapidly intermix, and the rising 

 vapour is therefore incessantly intermixing with the surrounding 

 air, yet when the temperature is high, the vapour must evidently 

 be liberated faster than the dispersion by intermixture can pro- 

 ceed; and consequently the relative specific gravities of the 

 vapour and surrounding air tend much to suppress or elevate the 

 vapour. A heavier air would accelerate the ascent of the vapour, 

 and by this means contribute to diminish the recondensation ; 

 and hence increase the apparent evaporation. But an air as 

 light or lighter than the vapour would not favour the vapour's 

 ascent, and would, therefore, diminish the apparent evaporation 

 by increasing the recondensation. Consequently if the evapo- 

 rating fluid, its temperature, and the pressure of the superincum- 

 bent air be the same, the greater the specific gravity of the surround- 

 ing air, provided it exceed that of the vapour, the greater the 

 apparent evaporation. 



I am not aware that this phamomenon has ever been noticed ; 

 but if philosophers who may be engaged on this part of physics 

 would have the goodness to attend to these hints, they may pro- 

 bably unfold an interesting set of laws, and add something 

 perhaps important to our present stock of knowledge. 



Several ways have occurred to me of evolving the temperature 

 in a function of the quantity of water left, supposing evaporation 

 alone to affect the temperature, and that at a given temperature 

 there was a given quantity of water ; but the utter defect of 

 experiments again prevents my proceeding. However the 

 following views of the subject rust on principles so simple and 

 susceptible of modification, if experiments should require it, that 

 I am induced to give them a place here, if it be only to excite 

 philosophers to present us with some experiments on this inte- 

 resting part of evaporation. Though in my theory of evaporation 

 I have said that the vapour arises from a decomposition at the 

 surface, it is evident that no theoretical views only can furnish 



