Suitable high polymers exhibiting the string-drawing phenomenon include polyvinyl alcohol, methyl cellulose, carboxymethyl cellulose, alcoholic solutions of nitrocellulose and proteinaceous substances, such as albumin, gelatin, and the extract of the roots of Hibiscus manihot L. The high polymer substances should preferably be present in the solution in an amount ranging from 0.0050.1 percent by w