394 



HOW CROPS GROW. 



vessel of water. He saw the liquid rise in the narrow 

 tube and fall in the outer vessel. He designated the 

 passage of water into the funnel as endosmose, or inward 

 propulsion. At the same time he found the water sur- 

 rounding the funnel to acquire the taste of salt. The 

 outward transfer of salt was his exosmose. The more 

 general word, Osmose, expresses both phenomena ; we 

 may, however, employ Dutrochet's 

 terms to designate the direction of 

 osmose. 



Osmometer. When the apparatus 

 employed by Dutrochet is so con- 

 structed that the diameter of the nar- 

 row tube has a known relation to, is, 

 for example, exactly one-tenth that of 

 the membrane, and the narrow tube 

 itself is provided with a millimeter 

 scale, we have the Osmometer of Grah- 

 am, Fig 67. The ascent or descent of 

 the liquid in the tube gives a measure 

 of the amount of osmose, provided the 

 hydrostatic pressure is counterpoised 

 by making the level of the liquid with- 

 in and without equal, for which pur- 

 pose water is poured into or removed from the outer ves- 

 sel. Graham designates the increase of volume in the 

 osmometer as 'positive osmose, or simply osmose, and dis- 

 tinguishes the fall of liquid in the narrow tube as nega- 

 tive osmose. 



In the figure, the external vessel is intended for the reception of 

 \vater. The funnel-shaped interior vessel is closed below with mem- 

 bran^, and stands upon a shelf of perforated zinc for support. The 

 graduated tube fits the neck of the funnel by a ground joint. 



Action of the Membrane. When an attraction exists 

 the membrane itself and one or more of the substances 

 between which it is interposed, then the rate, amount, 

 and even direction, of diffusion may be greatly changed, 



