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



"We see here another beautiful accordance of our theory with 

 facts, and on a subject that, perhaps, philosophers would hardly 

 expect. 



It has been supposed that the evaporation is strictly propor- 

 tional to the tension, but the preceding table evidently shows, 

 that the formula I have given involves the true theory. When 

 experiments on evaporation are carried to much lower tempera- 

 tures, or made in atmospheres of high temperatures, it is neces- 

 sary to make an allowance for the proper tension of the vapour 

 in the atmosphere. Mr. Dalton it seems made his experiments 

 in temperatures too low to produce any sensible effect on the 

 numbers of the preceding table ; but in carrying the comparison 

 to experiments of much inferior temperatures, the computations 

 would generally come out higher than experiments. 



If we want a theorem to express the evaporation, taking 

 into account the condensation from the aqueous vapour in the 

 atmosphere, and supposing of course that no part of the eva- 

 porated vapour recondenses on the water, or that, if it does, 

 it is proportional to the quantity evaporated, our theory gives 



— ■ (T t — t E) where £ signifies the absolute evaporation in any 



given measure or weight at some fixed temperature t, for a 

 determinate small portion of time ; t' the tension of the vapour 

 at the fixed temperature t ; T the temperature of the water, t 

 the tension of its vapour at this temperature T ; T' the tempera- 

 ture of the atmosphere, and E the elasticity the vapour in the 

 atmosphere would have, were it to occupy at the same tempera- 

 ture T' the same space in a vacuum. 



This theorem is easily reduced to that I have given above ; 

 and by a little transposition many interesting things may be 

 shown to flow from it ; but I hasten to other matters. 



Cor. 2. — By the theory I have given, it appears that the incre- 

 ment of condensation depends exclusively on the temperature 

 and space occupied by the aqueous vapour ; and it is of no con- 

 sequence whether in that space there be none, little, or much, 

 of any other incondensible aeriform body. Hence follows what 

 has been thought a very singular property of vapours ; namely, 

 that mixed with any gas in a sufficient proportion they are 

 capable of supporting an indefinite weight, but alone only a 

 certain one according to the temperature. The complete solu- 

 tion of this paradox is, that water at a given temperature evapo- 

 rates at a given rate whatever be the superincumbent pressure ; 

 and vapour at a given temperature condenses at a rate inversely 

 proportional to the space through which it is diffused, without 

 any respect to the elasticity of the mixture of which it is a part. 

 Therefore when this space is too great, the evaporation of the 

 water, if there be any water, will gain on the condensation of 

 the gas, until the one balances the other, and then the vapour 

 will have attained its proper tension ; but if the space be too 



