Mr. de Luc 
408 
they can surmount the cause which keeps them disseminated 
in the space, namely, fire. The less the quantity of free 
fire (or of the cause of heat) in a space, the greater is the 
distance at which the particles of water have the faculty of 
uniting together, and also of following other tendencies, by 
abandoning their latent fire ; and vice versa. A final union 
of a sensible quantity of particles of water in the medium (or, 
what comes to the same, that precipitation of water which 
first appears in it as a mist) takes place, when the density 
of steam is arrived beyond its limits ; and those limits de- 
pend on temperature, because, the greater the quantity of 
free fire in the space, the nearer the particles of steam may 
come to one another, without a final union of their water. 
The whole in vacuo , as in air. Such is the physical principle 
common to hygrology and hygrometry. 
2. Hygroscopic substances are of three distinct classes. Some 
seize on the water of steam, by a chemical affinity with that 
liquid ; among these are acids, salts, and calces. Some only 
imbibe it by its tendency to propagate itself in capillary pores ; 
but, from their nature they receive no sensible increase in their 
bulk by its introduction ; in the number of these are porous 
stones. Lastly, some substances, which also only imbibe a 
certain quantity of water, are thereby expanded ; and these 
are most of the solids belonging to the vegetable and ani- 
mal kingdoms. Various hygroscopic phenomena, which only 
depend on the different properties of the substances them- 
selves, being thus foreign to the fundamental laws of hygro- 
metry, I shall here confine myself to the last class, which ap- 
pears the only proper one for that general purpose ; and, 
among the hygroscopes of that class, I shall only consider 
