4 o6 Mr. de Luc 
with the air according to its own laws, as if there were no 
air. 
The whole theory of hygrology, as far as I have been able 
to collect the phaenomena of that class, appears to me com- 
prehended in the foregoing propositions, founded on facts. 
The objects of that science are in general the cause of eva- 
poration, and the modifications of the evaporated water. The 
common source of the water thus disseminated in the atmo- 
sphere, is the surface of the earth ; whence, in spontaneous 
evaporation, both in air and in vacuo , as well as in ebullition, 
we see that water fly off with latent fire. If we collect that 
product in a close space, it acts in the same manner as a new 
quantity of expansive fluid. We know from experience, 
that an expansive fluid is really produced by ebullition, and by 
evaporation in an exhausted vessel : there is no reason why 
the cause of evaporation, and its product, should change in 
any case, only by the presence of air ; and in examining 
what may happen in open air, we find no particular cause of 
the destruction of that expansible fluid, nor any difficulty in 
conceiving its dissemination in every part of the atmo- 
sphere. 
But here we lose sight of steam, for it is as transparent as 
air itself : here also its mechanical action is as little per- 
ceivable as that of any set of scattered particles of air ; and 
though its specific gravity is much less than that of air, its 
quantity existing in the atmosphere is most times so incon- 
siderable, that it can hardly be discovered by that means, on 
account of other causes which also affect the specific gravity 
of a given mass of free air. Therefore, notwithstanding ouf 
