**Procedure**

The cellophane tubings were filled with distilled water and soaked in distilled water for 24 hours. The tubings were then soaked in turn, for 30 min each time in 50% ethanol, absolute ethanol and diethyl ether successively. The tubings were thereafter soaked in the mixture of benzene, acetic anhydride, and pyridine, prepared as described above, for 18 hours. Each tubing was then properly rinsed in distilled water and stored in 10% KCl solution at 4°C until required.
