ABSORPTION 



147 



plant sources. Thus there is parchment (animal skin), formerly much used 

 by bookbinders and still obtainable from them; animal bladder, obtainable 

 sometimes in the fresh state, and always in a cleaned and dry state from 

 supply companies at a cost of about 10 cents each; gut membranes, in which 

 sausages are placed, having the advantage that they may readily be used in 

 the cylindrical form; egg membrane (lining the shell of an egg), which is diffi- 

 cult to remove uninjured by pulling off the shell, but which can be obtained 

 to perfection by dissolving away the shell in hydrochloric acid of about half 

 strength; and, finally, parchment paper (a tough paper so treated with sul- 

 phuric acid that the fibers are partially dissolved and allowed to melt together 

 again without appreciable openings), obtainable at a very low cost from 

 all supply companies, either in sheets or in cylinders of indefinite length 

 and 40 mm. diameter. It is either with the animal bladder or with parch- 

 ment paper that much of the earlier work upon osmosis (or dialysis) was 

 done. Yet another kind, extremely useful for demonstration purposes, 

 is now available in the diffusion shells (cylindrical round-bottomed cups) 

 of ^CHLEICHER and SCHUELT.. They are of two sizes, both 10 cm. long, 

 but of 16 mm. and 40 mm. diameter respectively, obtainable from supply 

 companies at a cost of about 25 cents each. Another and very different 

 kind of membrane is that formed by films of collodion, which may be made 

 into cups or tubes, a substitute for the diffusion shells, by the method 

 described by KARL KELLERMAN (in Journal of Applied Microscopy, 5, 1902, 

 p. 2038). In synopsis the method is this: into a clean test-tube pour enough 

 3% collodion to coat the tube when it is inclined and rotated; when the 

 collodion begins to dry, rest the test-tube inverted over a screen or blocks 

 so the excess may drain off and the remainder may dry; let it stand for 

 5 minutes to an hour, then fill with water, when the tube will loosen and 

 may be drawn out. Longer drying makes a tougher tube but an osmotic- 

 ally slower one, as does, of course, greater thickness or a thicker solution 

 of collodion. Such tubes will keep for some time in water, though they 

 become brittle in air. They form excellent, though very slow, osmotic 

 membranes, and hence should be used as thin as practicable. 



The student should now apply his knowledge, gained in 

 connection with the immediately preceding experiments, to an 

 interpretation of the facts of osmosis here observed, and should 

 follow the subject in the literature until he understands what 

 is known as to the physical forces producing the entrance of the 

 water against the increasing hydrostatic pressure, and what deter- 

 mines the limit of the rise. 



While the preceding experiment demonstrates an osmotic 

 absorption of water under conditions somewhat resembling 

 those in root-hairs, at the same time it shows one great difference, 

 namely, the solution escapes through the membrane, something 



