Mr. de Luc 
410 
a fixed module for determining correspondent degrees ; but 
these modules are not of the same nature ; and thence, in their 
relation to one another, both in the whole and in correspon- 
dent parts, moisture assumes in the medium, the character of 
a cause, and in hygroscopic bodies, that of an effect, as will 
appear by the following determination of both cases. 
6. Moisture is totally absent, first in the medium, when it 
contains no steam ; then (as a consequence) in hygroscopic 
bodies, because they contain no more water than can evapo- 
rate, without a decomposition of their component parts. The 
case supposed in that definition, is when, by some adequate 
cause, no sensible quantity of steam is permitted to remain in 
the medium ; as in my lime-vessel. Moisture is extreme, first 
in the medium, either air or a space free from air, when no 
more steam could be introduced into it, without a part of it 
being decomposed ; and then (also as a consequence to be ex- 
plained) it is extreme in hygroscopic bodies, because no more 
water can be admitted into their pores. Now, from the na- 
ture of the last of those maxima, the quantity of water which 
produces it in a given body is fixed, since it is determined 
by the actual capacity of its pores ; whereas the quantity of 
water which produces extreme moisture, in a medium of a 
given extent, is as variable as the temperature: therefore, 
the hygroscopic equilibrium between the medium and hygro- 
scopic bodies in different stages of moisture, which equili- 
brium is the object of hygrometry as a science, does not de- 
pend on certain quantities of water contained in the first, of 
which the last may receive their share ; it depends on diffe- 
rent aptitudes of the steam contained in the medium to com- 
municate water to those bodies ; which aptitudes vary, not 
