EXPERIMENTS on EVAPORATION. 133 
we have mentioned as one of the caufes of evaporation. 
The procefs alluded to is that by which moifture is collect- 
ed on the furfaces of cold bodies expofed to warm air—Dr. 
Franklin has explained this upon the principle that the wa- 
ter in the atmofphere is combined with heat, and that it is 
collected on the cold furface in confequence of the paflage 
of this heat into the cold body. This explanation is the 
reverfe of that which I have adopted, and as it explains to 
the fatisfaction of every one, a procefs the reverfe of eva- 
poration, it ftrengthens that explanation. 
When confidering.this theory of our great philofopher, 
and the pleafing application of it tomany important procefles 
of nature, itoccurred to me totry the converfe of the propo- 
fition; for if the colletion of moifture on the furface of a 
body depends upon the abftraGion of heat from the air by 
it, it follows, that when a body is not in a condition to 
receive heat from the atmofphere, no moiflure can collec 
upon it. 
As mirrors fhow the prefence of moifture with fo much 
accuracy | heated one of them, and found that although, 
when below 98°, they are covered with mift, if expofed 
but a moment to the breath, yet when heated but little a- 
bove 98°, Icauld not imprefs any moifture upon it, al- 
though it was applied clofe to my mouth and breathed up-~ 
on very frequently. Dry. Franklin’s propofition requires 
nothing toconfirm it, but if it were doubtful, this lat 
experiment would furnifh a ftrong argument in its 
favour. 
