TEST PAPERS IN PHOTOGRAPHY.
By DAVID GRAY ARCHIBALD.

HE use of litmus for indicating the acidity or alkalinity of fluids is known to every photographer. But the exact meaning of the chemical action that takes place when test papers or solution turn red or blue in a bath is not very clear. Most of us know that if the test paper turns red the solution is acid and if it turns blue it is alkaline. The preparation of litmus for testing purposes, whether on paper or in tincture does not concern us as practical photographers. Those so inclined can find full particulars of the methods of preparing it in any standard work on chemistry. The usual way of getting it is in strips or books. It is the better for being kept in bottles, closely stoppered and out of the light. Light action fades it and its color should be uniform and neither too light nor too dark. Litmus paper comes in red and blue, and most photographic dealers sell it in ten cent packets.

Naturally the color of litmus is red and it only becomes blue from the action of a free alkali.

In making up toning baths, etc., its use is a guide to the condition of our bath. The gold chloride used in toning comes in an acid condition and we add a solution of an alkali to a mixture of it to bring our bath to a state slightly alkaline, and here is where the rub comes in. This litmus paper is not nearly so sensitive a test as is commonly supposed, although if given time it will react if a bath be slightly alkaline or acid. If it turns rapidly red or blue, we have no knowledge of certainty how acid or alkaline the solution is. By making up our solutions long before we want them it is given time to react. If a solution is too acid or too alkaline we can add an alkali or an acid and bring it around again. One already affected strip in the bath will show the change that takes place.

There is another test paper than litmus although not as much used. This is turmeric paper. It is obtainable commercially and its change of color in the presence of an alkali from a yellow to brown is highly characteristic and perceptible.

Much testing will not be necessary if we proceed in a systematic way, not hap-hazard manner, in making up our baths. Add definite quantities of chemicals in making up baths. Then a test to be sure we are right and we are ready to proceed.