RELATION OF VAPOUR AND AIR. 567 



less distinctness, to several of the physical philoso- 

 phers of whom we have spoken. They were aware 

 that the vapour 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 atmosphere, to which, if we depress 

 bodies, water forms upon them in fine drops like 

 dew; this temperature is hence called the dew- 

 point. The vapour of water which exists anywhere 

 may be reduced below the degree of heat which is 

 necessary to constitute it vapour, and thus it ceases 

 to be so. Hence this temperature is also called the 

 constituent temperature. This was generally known 

 to the meteorological speculators of the last cen- 

 tury, although, in England, attention was princi- 

 pally called to it by Dr. Wells's Essay on Dem, in 

 1814. This doctrine readily explains how the cold 

 produced by rarefaction of air, descending below the 

 constituent temperature of the contained vapour, 

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

 refutes one obvious objection to the theory of inde- 

 pendent vapour. 



The other difficulty was first fully removed by 

 Mr. Dalton. When his attention was drawn to the 

 subject of vapour, he saw insurmountable objections 

 to the doctrine of a chemical union of water and 

 air. In fact, this doctrine was a mere nominal ex- 

 planation; for, on closer examination, no chemical 

 analogies supported it. After some reflection, and 



