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In like manner when fmoak or fleam ilTuing from 
the pipe of a boiling veffel firft rifes into the air, it 
appears in curled wreaths, and renders the air opake, 
but as foon as it is intirely difperfed, the tranfparency 
is reftored. Thus alfo in a calm hot fun-fhine day, 
when we look along a moift piece of ground, the 
air, and any objedt feen through it, appears to have a 
tremulous motion like that which we obferve in an 
objedt feen through any two fluids that are mixing 
together. 
Now as the vapours rife here in great abundance, 
and the air has but little motion, thofe parts of it 
that are much impregnated with the aqueous parti- 
cles are mixed gradually with the air that is drier 
and of a different denlity, which will occaflon re- 
fradlions of the light, and that apparent tremulous 
motion jufl: now mentioned. And in this cafe the 
folution of water in air (if I may yet call it fo) is 
carried on in a manner viflble to the eye, as it is in 
other fluids. The fame tremulous undulating mo- 
tion is more obfervable when we look in warm wea- 
ther through a telefcope, which magnifies the va- 
pours floating in the air j and from this kind of re- 
fraftion the twinkling of the flats feems to arife, 
with this difference only, that the watery refradling 
particles in the day time are pafling into a flate of 
folution, wherea-s the vapours already diffolved are, by 
the cold of the night, beginning to precipitate and 
return into particles large enough to caufe refradlions 
in the light of the flars. 
Thirdly ; Heat promotes, and cold in fome mea- 
fure flops, or checks, both folution and evaporation ; 
very hot water will diflblve fait fooner, and in a greater 
X 2 quantity 
