18 GENERAL PROPERTIES OF LIVING TISSUES 



diffusion bulb twenty-four hours in running 

 water. Dry the bulb and coat its neck inside 

 and out with paraffin; when this has become 

 firm, fill the bulb to above the lower edge of the 

 paraffin with solution of copper sulphate (2.5 gm. 

 per litre). Place the bulb in a beaker and pour 

 in a solution of potassium ferrocyanide (2.1 gm. 

 per litre) until the lower edge of the paraffin is 

 covered. Keep the bulb in this solution over 

 night. Where the two solutions meet within the 

 clay wall a precipitation membrane of copper 

 ferrocyanide will form. This membrane is sup- 

 ported by the clay wall. Pour out the contents 

 of the bulb and rinse with cold distilled water. 

 Through such precipitation membranes water 

 and some other solvents will readily pass, while 

 many salts dissolved in the solvent are kept 

 back. 



Fill the bulb with one per cent solution of 

 cane sugar. Insert in the neck of the bulb a 

 tightly fitting rubber stopper pierced by a small- 

 bore glass tube about ten feet long. Stand the 

 bulb in distilled water and support the long 

 tube in suitable clamps. The sugar will not pass 

 out through the membrane, but water will pass 

 through it into the bulb, and the solution will 

 rise in the tube at the rate of several inches an 

 hour. The rate is slow because the friction in 



