848 HOW CROPS GROW. 



Those vegetable substances which ordinarily manifest 

 the greatest absorbent power for water, are pectin, pectic 

 and pectosic acids, vegetable mucilage, bassorin, and al- 

 bumin. In the living plant the protoplasmic membrane 

 exhibits great absorbent power. Of mineral matters, 

 gelatinous silica (Exp. 58, p. 123) is remarkable on account 

 of its attraction for water. 



Not only do different substances thus exhibit unlike ad- 

 hesion to water, but the same substance deports itself va- 

 riously towards different liquids. 



100 parts of dry ox-bladder were found by Liebig to 

 absorb during 24 hours : 



268 parts of pure Water. 

 133 " " Saturated brine. 

 38 " Alcohol (84 1 .) 

 17 " " Bone-oil. 



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 ex- 

 perience with greased boots. India-rubber, on the other 

 hand, is impenetrable to water, while oil of turpentine is 

 imbibed by it in large quantity, causing the caoutchouc 

 to swell up to a pasty mass many times its original 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 is 

 shown by what has been learned from the study of a 

 kind of pores whose effect admits of accurate measure- 

 ment. A tube of glass, with a narrow, uniform caliber, is 

 such a pore. In a tube of 1 millimeter, (about ^ of an 

 inch) in diameter, water rises 30 mm. In a tube of T ' ff mil- 

 limeter, the liquid ascends 300 mm., (about 11 inches) ; 

 and in a tube of T * ff mm. a column of 3,000 mm. is sus- 

 tained. In porous bodies, like chalk, plaster stucco, closely 

 packed ashes or starch, Jainiii found that water was 



