CAUSES OF THE MOTION OF JUICES. 387 



sap-wood of trees is far more absorbent than the heart- 

 wood and bark. The cuticle of the leaf is often com- 

 paratiTely impervious to water. Of the proximate ele- 

 ments we have cellulose and starch-grains able to retain, 

 even when air-dry, 10 to 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, unabsorbent. 



Those vegetable substances which ordinarily manifest 

 the greatest absorbent power for water, are the gummy 

 carbhydrates and the albuminoids. In the living plant 

 the protoplasmic membrane exhibits great absorbent 

 power. Of mineral matters, gelatinous silica (Exp. 58, 

 p. 137) is remarkable on account of its attraction for 

 water. 



Not only do different substances thus exhibit unlike 

 adhesion to water, but the same substance deports itself 

 variously towards different liquids. 



. One hundred parts of dry ox-bladder were found by 

 Liebig to absorb during 24 hours : — 



A piece of dry leather will absorb either oil or water, 

 and apparently with equal avidity. If, however, oiled 

 leather be immersed in water, the oil is gradually and 

 perfectly displaced, as the farmer well knows from, his 

 experience with greased boots. India-rubber, on the 

 other hand, is impenetrable to water, while oil of tur- 

 pentine is imbibed by it in large quantity, causing the 

 caoutchouc to swell up to a pasty mass many times its 

 ojig:inal bulk. 



The absorbent power is influenced by the size of the 

 pores. Other things being equal, the finer these are, the 

 greater the force with which a liquid is imbibed. This 



