2 Mr. de Luc on 
effects produced in it by moijiure were proportional to the in- 
tenfities of that caufe ; and, confequently, that a true hygro- 
metrical fcale was to be a particular objedt of inquiry (§ 2.). 
4th, laftly, That perhaps the comparative changes, of the 
dimenjions of a fubftance, and of the weight of the fame or 
other fubftance, by the fame variations of moijiure , might lead 
to fome difcovery m that refpect (§ 72.). The fame propoft- 
tious will be the fubjedt of this Paper. 
Of abfolute drynefs. 
1. An hygrofcopic body, which is not brought into contadt 
with any other body drier than itfelf, cannot lofe any part of 
its moijiure but by evaporation ; and if this is iutirely produced 
by fire, there may be fuch a degree of heat as will caufe the 
total evaporation of that moijiure. 7 his is the principle on 
which the above firft propofitiou was founded ; but at the fame 
time I mentioned, that I had not brought it into pradtice, be- 
caufe of the impoffibility of fubmitting the fubftance of the 
hygrometer to fuch a degree of heat. However, I foon removed 
that difficulty by confidering, that the degree of heat neceffary 
to produce extreme drynefs , might be applied to fome fubftance 
that could bear it ; and that drynefs be tranfmitted to the hygro- 
meter, by inclofing it with that fubftance in a proper veffel. 
The fubftance I chofe was potaf ; and I prepared, for this and 
fome other hygrometrical purpofes, an apparatus which w r as 
made by Meff. Nairne and Blunt in >776. But anew 
objedtion flopped me again in that purfuit, and led me for fome 
time to a very great and now almoft ufelefs labour. The 
degree of drynefs produced by potajh fo ufed, could be only 
proportionable to the degree of heat that it had received ; and 
not 
3 
