Hygrometry . 
put into it an hygrometer, whofe fixed point of drynefs has been 
taken in the lime apparatus : and in five weeks it was fixed to 
the fame point. This is a demonftration, that the nature of the 
fubftance does not interfere with the degree of drynefs produced, 
and that incandefcence is the only caufe of its fixity. 
1 1. Laftly, in refpefit of hygrometry , a degree of drynefs thus 
determined might have been fuffipent ; but for hygrology , 
and even for natural phiiofophy in general, it was definable to 
difcover if that fixed degree of drynefs was alfo abfolute : and the 
following are the confiderations which directed me in that en- 
quiry : if evaporation is produced by heat only, and if incan- 
defcence is the maximum of heat ; an hygrofcopic body, which 
is brought to incandefcence, cannot contain any evaporable water; 
and if that body has fuch a mafs, as to be capable of abforbing 
all the water evaporated in a certain fpace, without acquiring any 
meafurable moiflure , that fpace may be called abfolutely dry . 
Now, if an hygrofcopic fubftance which is inclofed in that fpace, 
contains any fenfible quantity of evaporable water, when heat 
increafes, that fubftance muft lofie a part of its moifiure in the 
medium , and take it back by the diminution of heat. Con fie- 
quently there was a means of difccvering, if hygrofcopicfabikzttQQs, 
reduced to the above degree of drynefs , ftiil retain a fenfible 
quantity of evaporable water ; it was that of obferving their 
weight, by changes of heat : and from thefe previous confiderations 
I made the following experiments. 
12. I hung fucceffively to a very fenfible beam , (hewing the 
changes of weight by an index, different forts of vegetable and 
animal fubftances, the beam being inclofed in a glazed tin- 
vefiel, containing a fufficient quantity of quicklime ; and dur- 
ing the operations, I produced from time to time great changes 
in the temperature of the veflel. As long as thefe fubftances 
Vol. LXXXI. C retained 
