388 



HOW CROPS GROW. 



is stown by what has been learned from the study of a 

 kind of pores whose effect admits of accurate measure- 

 njent. 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 millimeter, the liquid ascends 300 mm. (about 11 

 inches) ; and, in a tube of -^^-^ mm., a column of 3,000 

 mm. is sustained. In porous bodies, like chalk, plaster 

 stucco, closely packed ashes or starch, Jamin found that 

 water was absorbed with force enough to overcome the 

 pressure of the atmosphere from three to six times ; in 

 other words, to sustain a column of water in a wide 

 tube 100 to 200 ft. high. {Gomptes Rendus, 50, p. 311.) 



Absorbent power is influenced by temperature. Warm 

 water is absorbed by wood more quickly aud abundantly 

 than cold. In cold water starch does not swell to any 

 striking or ev«n perceptible degree, although consider- 

 able liquid is imbibed. In hot water, however, the case 

 is remarkably altered. The starch-grains are forcibly 

 burst open, and a paste or jelly is formed that holds 

 many times its weight of water. (Exp. 27, p. 51. ) On 

 freezing, the particles of water are mostly withdrawn 

 from their adhesion to the starch. The ascent of liquids 

 in narrow tubes whose walls are unabsorbent, is, on the 

 contrary, diminished by a rise of temperature. 



Adhesive Attraction.— The absorption of a liquid 

 into the cavities of a porous body, as well as its rise in a 

 narrow tube, are expressions of the general fact that 

 there is an attraction between the molecules of the liquid 

 and the solid. In its simplest manifestation this attrac- 

 tion exhibits itself as Adhesion, anrl this term we shall 

 , employ to designate the kind of force under considera- 

 tion. If a clean plate of glass be dipped in water, the 

 liquid touches, and sticks to, the glass. On withdraw- 

 ing the glass, a film of water comes away with it — ^the 

 adhesive force of water to glass being greater than the 

 cohesive force among the water molecules. 



