4 02 Mr. de Luc on 
without any precipitation of water from the medium. This I 
have produced by means of a wire cage, 4 inches in diameter, 
covered with cotton cloth, having at the top a refervoir , by 
which the cloth is kept thoroughly wet for a long time ; which 
cage befides is inclofed in the glafs jar inverted over water. In 
that apparatus, though in fummer-time, every hygrometer , 
either thread or Jlip, moves and fixes itfelf, not fo fpeedily, but 
elfe exactly as if it was plunged into water , without any fuper - 
faturation of the inclofed medium , or precipitation of water on 
the hygrofcopic fubftance. 
81. We may fee now that the idea of two forts of extreme 
moijiure is without any foundation. In order to enforce the 
necefiity of taking the point of extreme moijiure in the air 
brought to that ftate, and not in water, M. de Saussure fays, 
44 That the hygrometer is not to meafure the moijiure of water , 
44 but that of the air This at firfl appears plaufible ; how- 
ever, in reality, moijiure is no more to be considered in water 
itfelf, than heat in the fluid called fire. Water is the caufe of 
moijiure , as fire is the caufe of heat ; but thofe effeSs are not 
produced on their caufes ; it is on other fubftances. Therefore, 
if fome hygrofcopic fubftances are placed in a medium which has 
attained extreme moijiure , and, in proportion as they take 
water from it, the lofs of that water be conftantly repaired 
by a new evaporation , they will receive by degrees in fuch a 
medium , without any precipitation , as much water as if they 
were plunged in water itfelf; for the limit is their capacity, 
which I have explained in § 19. This is the fame theory that 
1 had expreffed in my firfl: Paper on Hygrometry ; and it is com- 
pletely confirmed by the above experiment, with the difcovery 
of this new hygrofcopic law : 64 That in a Jlagnant air, every 
44 evaporating furface has an atmofphere of extreme moijiure, 
44 which 
