Page:Paper and Its Uses.djvu/115

Rh spirit. Place the test tube in a beaker containing water, and heat slowly. The spirit will boil before the water reaches boiling point, and in a short time the resin will be dissolved. As soon as the solution is cool, pour it into a test tube half full of distilled water, and the resin will appear as a ring, whitish in colour, at the junction of the two liquids. If the test tube is shaken up the opalescent appearance of the liquid indicates the presence of resin.

Starch.—A very weak solution of iodine in potassium iodide is dropped on the paper with a glass rod, or a strip of the paper is dipped into the solution. If a very small quantity of starch is present in the paper a blue reaction will take place, and the larger the amount, the darker the coloration. Therefore in order to form an opinion as to the quantity of starch used, a very pale solution must be used, or the colour may be too dark to enable one to make comparisons. An aqueous extract of the paper may be treated with the iodine solution, and if a comparative test is to be made, it is necessary to work on each paper with identical quantities of water for boiling, and iodine solution for testing. A faint colour must not be taken as evidence of added starch, as in rag pulp it is very difficult to remove starch from the raw materials.

Colouring Matters.—The tests for colours should be for the purpose of discovering whether they are reasonably fast to light and when wetted. The first can be tested by exposing the paper to a steady light—not sunlight—for a period according to the time of year. A photographic printing frame with a black disc on plain glass is a convenient method of testing, and if, after forty-eight hours in summer, and a proportionately longer period at other times, the difference between the exposed and protected areas is marked,