CAUSES OP THE MOTION' OP JUICES. 347 



their pores, but they forcibly drmk 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 imbibins^- 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 gi'anite 

 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- 

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