An untoned print is not so permanent as one that has been toned; indeed, the object of toning is to protect the easily affected silver in the print by coating it with a metal that is better able to resist adverse influences. The value of toning may be demonstrated by the following experiment. Prepare some sulphuretted hydrogen water (SH2) by placing a small piece of iron sulphide in a test tube A (see sketch), half filling it with water, and adding a little sulphuric acid. Pit a cork B, bored to take a length of glass tube C. Pit up a glass flask with tubes D and E, and nearly till with distilled water. Connect D and C with a length of rubber tubing. Now warm the test tube in a gas flame, and the gas will readily be driven off through the tubes, and eventually bubble through the water in which it dissolves. Allow the action to continue spontaneously for an hour. This operation should be performed out of doors. Cut in halves two prints - one that has been toned a decided blue, and the other untoned. Place one half of each print in the SH2 obtained from the flask. Both pieces will become lighter, and will be altered in appearance, as will be seen on comparison with the untreated halves.

On removal from the SH2, the toned print will be found to have faded equally with the other, but will be less altered otherwise.