170 HISTORY OF THERMOTICS. 



Sect. 3. Dal ton's Doctrine of Evaporation. 



A PORTION of that which appears to be the true notion of evaporation 

 was known, with greater or less distinctness, to several of the physica. 

 philosophers of whom we have spoken. They were aware that the 

 vapor which exists in air, in an invisible state, may be 'condensed into 

 water by cold : and they had noticed that, in any state of the atmo- 

 sphere, there is a certain temperature lower than that of the atmo- 

 sphere, to which, if we depress bodies, water forms upon them in fine 

 drops like dew ; this temperature is thence called the dew-point. The 

 vapor of water which exists anywhere may be reduced below the 

 degree of heat which is necessary to constitute it vapor, and thus it 

 ceases to be vapor. Hence this temperature is also called the consti- 

 tuent temperature. This was generally known to the meteorological 

 speculators of the last century, although, in England, attention was 

 principally called to it by Dr. Wells's Essay on Dew, in 1814. This 

 doctrine readily explains how the cold produced by rarefaction of air, 

 descending below the constituent temperature of the contained vapor, 

 may precipitate a dew ; and thus, as we have said, refutes one obvious 

 objection to the theory of independent vapor. 



The other difficulty was first fully removed by Mr. Dalton. \Vheu 

 his attention was drawn to the subject of vapor, he saw insurmountable 

 objections to the doctrine of a chemical union of water and air. In 

 fact, this doctrine was a mere nominal explanation ; for, on closer ex- 

 amination, no chemical analogies supported it. After some reflection, 

 and in the sequel of other generalizations concerning gases, he was led 

 to the persuasion, that when air and steam are mixed together, each 

 follows its separate laws of equilibrium, the particles of each being- 

 elastic with regard to those of their own kind only : so that steam 

 may be conceived as flowing among the particles of air 12 " like a 

 stream of water among pebbles ;" and the resistance which air offers 

 to evaporation arises, not from its weight, but from the inertia of its 

 particles. 



It will be found that the theory of independent vapor, understood 

 with these conditions, will include all the facts of the case ; gradual 

 evaporation in air; sudden evaporation in a vacuum; the increase of 



12 Manchester Memoirs, vol. v. p. 58 1 



