i4 Mr. de Luc on 
to feveral hygrofcopic fubftances, each of them receives its 
maximum ; which is determined by the equilibrium , between 
their total capillary power , and the refiftance of their pores to be 
more dilated. That final equilibrium , which, from its very nature, 
cannot be overpaffed in any elajlic fubftance properly ufed, 
determines the fpecific capacity of thofe fubftances for moijiure. 
20. Moijiure then, confidered in porous bodies not Joluble 
by water , may be defined, u A quantity of water , which is 
“ invijibly contained in their pores ; without any other connec- 
“ tion with their fubftance, than that which it has with the 
46 glafs of the capillary tubes into which it has afeended.” 
21. We may fee now whence proceeds the hygrofcopic equili- 
brium between elaftic fubftances inclofed in a fpace, either 
filled with air or deprived of it. In this explanation it is 
unneceflary to determine, how water is invifbly difleminated in 
fpaces free from vifible bodies, therefore I (hall not enter here 
into this fubjedl ; that diffemination is a fa£t admitted in every 
hypothefis, confequently the medium is only to be confidered as 
the flock and ftandard of moifture. By the caufe, whatever, of 
evaporation , hygrofcopic fubftances lofe or gain water in the 
medium , according to its degree of moifture , till they are in 
equilibrium with it ; which implies the equilibrium amongft 
themfelves according to the laws refulting from their own 
nature. 
22. Now moifture , in a general fenfe, will appear to be, 66 a 
“ quantity of invifble water , either evaporable , or evaporated.” 
And from that definition, the maximum of moifture will exift, 
when, 66 every circumftance remaining the fame, no more 
64 water can be admitted in a fpace, without becoming vifible ; 
on folid bodies, by their furface being wet; and in the medium , 
4 “ by 
