CAUSES OF THE MOTION OF JUICES. 347 
their pores, but they forcibly drink in or absorb this liquid, 
when it is presented to them in excess, until their pores 
are full. 
When the molecules of the porous body have freedom 
of motion, they separate from each other on imbibing a 
liquid; the body itself swells. Even powdered glass or 
fine sand perceptibly increases in bulk by imbibing water. 
Clay swells much more. Gelatinous silica, pectin, gum 
tragacanth, and boiled starch, hold a vastly greater 
amount of water in their pores. 
In case of vegetable and animal tissues, or membranes, 
we find a greater or less degree of expansibility from the 
same cause, but here the structural connection of the 
molecules puts a limit to their separation, and the result 
of saturating them with a liquid is a state of turgidity 
and tension, which subsides to one of yielding flabbiness 
when the liquid is partially removed. 
The energy with which vegetable matters imbibe water 
may be gathered from a well-known fact. In granite 
quarries, long blocks of stone are split out by driving 
plugs of dry wood into holes drilled along the desired line 
of fracture and pouring water over the plugs. The liquid 
penetrates the wood with immense force, and the toughest 
rock is easily broken apart. 
The imbibing power of different tissues and vegetable 
matters is widely diverse. In general, the younger or- 
gans or parts take up water most readily and freely. The 
sap-wood of trees is far more absorbent than the heart- 
wood and bark. The cuticle of the leaf is often compara- 
tively impervious to water. Of the proximate elements 
we have cellulose and starch-grains able to retain, even 
when air-dry, 10-15°|, of water. Wax and the solid fats, 
as well as resins, on the contrary, do not greatly attract 
water, and cannot easily be wetted with it. They render 
cellulose, which has been impregnated with them, unab- 
sorbent. 
