The sorption of water by cellulose is one index of the fine structure of the gel. The sorption of water vapor is determined by two phases or stadia, continuous with each other. There is a primary surface or chemosorption, dependent on the free hydroxyls. Water molecules are held to these by strong forces, considerably greater than for the condensation of water vapor. There is also a volume sorption, or capillary condensation. Mercerization--i.e., treatment with caustic alkali solutions of 18% or higher, followed by washing out the alkali--gives a cellulose with a coarser lattice structure and higher water absorption at all vapor pressures. Similar materials are obtained by regeneration of cellulose from curammonium solution or from viscose, as also by hydrolysis of cellulose esters reprecipitated from acetone solution. These are termed hydrate celluloses. On the other hand, degrading treatments, such as acid tendering and oxidation, do not materially change the water adsorption. Wood pulp (alpha-pulp) has a higher sorption than cotton cellulose, approaching mercerized cellulose. But the sorption of water vapor is unaffected by beating ( hydration ). Hydration comprises effects depending upon increase of external surface or dispersity. This increase permits closer interlocking of the dried felt or sheet, hence, inter alia, lower absorption, and retention of liquid water thereby.