Mr. de Luc on Evaporation. 401 
every liquid cools when it evaporates ; for I considered that 
circumstance as a proof, that the portion of the liquid which 
then disappears, is carried away by a quantity of fire proceed- 
ing from the liquid itself. 
The general phenomena of evaporation and moisture had 
been my only guides, when I first published my opinion in 
that respect in my w r ork Rech. sur les Mod. de V Atmosphere : 
but it w r as then in a very imperfect state, and I owe much to 
Mr. Watt, the inventor of the new steam engine, for the 
degree of precision which that theory has acquired in my last 
w r ork, Idees sur la Meteorologie. In particular, I am indebted 
to that truly ingenious and learned experimental philosopher, 
for an immediate proof of my fundamental opinion, resulting 
from an experiment, that he was so good as to repeat in my 
presence, and which demonstrates, that in the common eva- 
poration of water in open air, the quantity of heat lost by 
the mass, bears to the quantity of water carried away, a pro- 
portion still greater than that which is found in the steam 
produced by boiling water. Therefore there is no reason to 
doubt, that steam is formed in the first, as in the last of 
those cases. 
It is true, that in some other respects, appearances are very 
different in those two classes of phaenomena ; but those diffe- 
rences proceed from well determined causes, as will appear 
by the following abstract of my theory. 
On the Laws of Hygrology. 
1. Whenever water is in a state of evaporation, an expan- 
sible fluid, composed of water and fire, is produced. To that 
