on Evaporation. 411 
only by tlie different densities of that fluid, but in steam of 
the same density according to the temperature. 
7. In order to assign the cause of that equilibrium, we have 
previously to determine, 1. Whence proceeds a maximum of 
length in the hygroscopic bodies here referred to ; 2. How 
that maximum is produced by steam ; 3. Why it takes place 
by different densities of that fluid, when the temperature is 
different. 
8. Water introduces itself into the above bodies by its 
property of adhering and propagating itself on their surface, 
and thereby of entering their capillary channels ; and those 
bodies resist its introduction, which expands them, by the 
tendency of their particles to remain united. This last ten- 
dency, from its own law, being less and less in proportion as 
the particles of the body are already less in contact with 
each other, it should appear, that water ought at last to 
divide them entirely : but, according also to the law of that 
propagation of water, the greater is its quantity already in- 
duced on the internal channels of the body, the less is the 
tendency of new particles to enter them ; and this last dimi- 
nution being more rapid than the former, there comes a point 
in which the two forces are in equilibrium ; which circum- 
stance determines the maximum of the introduction of water, 
and consequently that of the lengthening of the body. 
9. The maximum of length of those hygroscopic bodies 
is thus produced, whenever a sufficient quantity of water 
stands or floats round them, without having a stronger ten- 
dency elsewhere. Therefore that maximum is produced 
when such a body is immersed into water ; and it is also 
produced when the same substance stands in a space where 
