on Evaporation. 419 
The maximum of evaporation was then surely produced ; 
but it remained short 3,5 degrees from extreme moisture : 
this was in air. 
I could not have any indication of the quantity of pressure 
produced by the steam, mixed then with the air ; for a re- 
ceiver, equally pressed in and out, is not air tight. An ex- 
periment for determining that increase of pressure, must be 
made in a sealed vessel, as in Mr. de Saussure’s experiment, 
by which he found that important phsenomenon : but from 
that experiment we are sure, that if a barometer had been 
inclosed in the above receiver, being then sealed, the steam 
produced would have made it rise, at the same time that it 
affected the hygrometer. 
The pump was then worked ; after which the instruments 
were found as follows. 
7,15 26,0 97,0 44,75 
I shall make here a remark on the known phenomenon of 
the diminution of heat, when a space filled with air is sud- 
denly exhausted ; as in this case we have an analogy that will 
enhance the most natural explanation of that phenomenon. 
Free fire, the cause of heat, being an expansible fluid, as air 
and steam, is then carried away with them, notwithstanding 
its tenuity, and heat is diminished in the space till new fire is 
come through the vessel. Now, the same is observed in re- 
spect of moisture with a quick hygrometer, for it goes for 
an instant to dryness, while the quantity of steam carried 
away is repairing by a new evaporation. 
In the above case, moisture was lastly increased 0,5 degree ; 
yet the quantity of steam was less than before, and the cause 
