The _toning_ of Paper Positives is the part of the process which is likely to injure their stability; inasmuch as the finest results cannot easily be obtained without incurring _sulphuration_, and the action of Sulphur, if carried to any extent, has been shown to be detrimental. The point to be kept in view, is to alter the original structure of the image as little as possible in toning; and it is best to use Gold in preference to Sulphur as the colouring agent. On theoretical grounds, toning by an alkaline solution of Chloride of Gold (p. 132), and fixing by Ammonia, is the best process; but the employment of Sel d'or, which gives a more agreeable colour and has not been found practically to injure the image, will be generally preferred. In using _a single fixing and toning Bath_ the same object of working by Gold rather than by Sulphur may be best attained by maintaining the activity of the Bath by constant additions of Chloride of Gold. The prints which are _least stable_ are such as have been toned in _acid Hyposulphite Baths, without Gold;_ and the difficulty of preserving such pictures from becoming yellow in the half-tones is very great. Possibly a portion of the Sulphuretted Acid may unite with the Suboxide of Silver and cannot be removed by washing (see p. 158); but even if this be not the case, it is certain that no ordinary amount of care will obviate the occasional occurrence of fading, unless the Hyposulphite Bath be kept _neutral to test-paper_. And all those plans of toning in which Acetic or Hydrochloric Acid is mixed with Hyposulphite of Soda, and the Positive immersed whilst the liquid is in a milky state from precipitation of Sulphur, ought studiously to be avoided. It will be well also to avoid pushing the action of the fixing and toning Bath to its utmost limits, since practice and theory both teach us that the Positives which have been long in the Hyposulphite, and consequently show a tendency to yellowness in the light parts, are most liable to lose their half-tones on keeping. Photographic Prints are found often to _darken_ slightly in the course of years; and therefore by suspending the toning action at an earlier stage a margin is left for what some have termed "an improvement by time." The use of _Albuminized_ in preference to plain paper gives an advantage in protecting the image from oxidation; but if constantly exposed to moisture, a putrefactive decomposition of the animal matter may occur. The proper colour of the Albumen image being a _pale red_, the black tones should not be sought for on that variety of paper: their production, if Hyposulphite of Soda were used in toning, would probably imply an amount of Sulphuration which would more than counterbalance any advantage otherwise derivable from the Albumen.