into aurous chloride according to Davanne and Girard. And an old bath in this state, for the change takes place but slowly replaces one atom of silver with one atom of gold.
AuCl + Ag = AgCl + Au.
And the prints in the latter case are most vigorous and permanent. Finally, the toning baths become quite colourless and will not tone. Numerous formulae have been given for toning baths, and in many cases merely the quantities of the salts are altered. In many cases the new formulae are recommended as giving exceptionally fine tones or possessing some particular property which in many cases can be explained by the fact that the addition of more of one ingredient may convert the bath from a neutral to an alkaline one. Mercier in his valuable little treatise, " Virages et Fixages," formulates the following rules which are instructive.
i. When, in making a toning bath, ingredients more or less alkaline are used, the proportion of such ingredients necessary to decolorise the bath is less the more alkaline it is. Taking as a typical bath one containing chloride of gold i part, water 1,000 parts, the following are the necessary proportions of salts to decolorise the bath completely in six hours: caustic potash 07 parts, potassium bicarbonate 2 to 3 parts, sodium tungstate 12 to 18 parts, sodium phosphate 25 to 30 parts, borax 4 to 5 parts. When only slightly soluble substances are used, then the above rule should read : When a solution of chloride of gold is treated with a substance slightly alkaline and slightly soluble, the decoloration is more rapid the more alkaline the substance is.
2. If the salts employed are alkaline oxides, the bath decolorises and becomes fit for use the quicker the greater the quantity of salt employed.
3. All neutral or slightly alkaline toning baths used im-mediately they are colourless, tone very rapidly and tend to give violet-black tones.
4. All neutral or slightly alkaline toning baths lose their activity gradually; the more alkaline the bath the quicker this takes place.
5. Acid toning baths (those reddening litmus paper very slowly) do not become colourless when made with pure mineral