on Evaporation. 4,13 
certain hygroscopic body, is the determined quantity of wai- 
ter which it can retain in the medium, according to the actual 
ratio in the steam. Moisture in the medium is greater, and 
an hygroscope retains in it a greater quantity of water, when 
the actual quantity of steam bears a greater proportion with 
its maximum correspondent to the temperature ; and vice 
versa. Therefore the divisions of the instrument are meant 
to indicate, by every temperature, the actual distance of mois- 
ture in the medium, either from its zero, or from its maximum ; 
and that distance is the degree of moisture. But this is yet with- 
out any reference to the density of steam ; the determination 
of this having the temperature for one of its indispensable 
elements. 
13. From that derives the remarkable phenomenon of the 
effects produced on moisture in the medium, by the variations 
of temperature, though the quantity of steam, or evaporated, 
water, remains the same in the same space ; effects of which, 
we are informed by the variations of the hygroscope, and> 
which are the same by the absence or presence of air. Every 
change in the temperature, operates a correspondent change 
in the module of moisture ; or in a variable totum, of which, 
according to its magnitude, the actual quantity of steam is a 
greater or a smaller aliquot part ; that module being always 
the quantity of steam that could not be exceeded in the space, 
by the actual temperature, without the decomposition of part 
of it ; which is always extreme moisture in the medium. 
14. From that results lastly ; that if, besides being ac- 
quainted, by the hygroscope, of the degree of moisture in the 
medium, or of the ratio between the density of steam and its 
actual module, it is required to know the density of that 
mdccxcii. 3 H 
