ABSORPTION 



149 



use, of forming the membranes in clay supports, as described by Morse 

 in his paper cited under Literature. 



A method of demonstrating, by projection upon a screen, the formation 

 of these membranes to an audience, is described by Pfeffer in his paper 

 earlier cited (page 23, note). 



The notable rise of the liquid against hydrostatic pressure 

 in the preceding experiment implies an equivalent osmotic pres- 

 sure within the solution, and the question arises as to how great 

 this may be. The exact study of the subject is beset with experi- 

 mental difficulties, but the student may follow it by aid of the 

 following : 



Suggested Experiments. Some idea, though crude and inadequate, 

 of the osmotic pressure may be obtained by replacing the slender glass tube 

 of F in figure 38 by a simple pressure- 

 gauge; but as there is an escape from, as 

 well as absorption into, the cup, it is plain 

 that the full pressure cannot be measured 

 except when the membrane is semi-per- 

 meable. The parchment membranes, how- 

 ever, are all too weak and yielding to serve 

 as supports for such membranes (though 

 they can easily be made to burst by the 

 pressure), and for this purpose firm sup- 

 ports, such as clay cells, must be used. 

 The best arrangement by far, where 

 moderate pressures are concerned, is 

 Pfeffer's cell, an instrument which has 

 become classical since the great physical 

 importance of its results has been under- 

 stood, though for stronger pressures the 

 special modification of this used by Morse 

 is required. The student, if his time 

 permits, should now prepare a Pfeffer's 

 cell. He will find great difficulties to 

 overcome, but ample reward if he tri- 

 umphs. It is described and figured in 

 Pfeffer's 'Osmotische Untersuchungen " 

 (Leipzig, 1877, 22), and the essential 

 parts of the description, with the figure 

 of his final arrangement (Fig. 40), is in 

 Harper's "Scientific Memoirs," IV, 1899. 

 Another figure, with a very valuable account 

 of precautions necessary to successes in Goodale's "Physiological Botany," 

 227. I have found that an excellent substitute for Pfeffer's cups, which 



Fig. 40. — Pfeffer's arrange- 

 ment FOR THE STUDY OF OS- 

 MOTIC pressures; figure re- 

 duced TO f. 



(From Pfeffer's " Osmotische 

 Untersuchungen .' ' ) 



