If Towgood's or any English paper be used, the Citric Acid, Carbonate of Soda, and Gelatine may be omitted. With a foreign paper the Citrate tends to give a purple tone to the Positive, when toned by Sel d'or, but the gold toning Bath must be in active order, or the prints will be too red. The Citric Acid also should not be in excess over the alkaline Carbonate. Render sensitive by floating for three minutes upon a Nitrate Bath of sixty grains to the ounce of water. Formula III. _Ammonio-Nitrate Paper._--This is always prepared without Albumen, which is dissolved by Ammonio-Nitrate of Silver. Take of Chloride of Ammonium 100 grains. Citrate of Soda 200 " Gelatine 20 " Water 20 fluid ounces. Dissolve the Gelatine by the aid of heat; add the other ingredients, and filter. The solution cannot be kept longer than two or three weeks without becoming mouldy. The Saxony paper, or Towgood's English paper, may be employed; the Gelatine and Citrate being retained or omitted, according to the taste of the operator and the mode of toning which is adopted. Render sensitive by a solution of Ammonio-Nitrate of Silver, 60 grains to the ounce of water, which is prepared as follows:-- Dissolve the Nitrate of Silver in one-half of the total quantity of water. Then take a pure solution of Ammonia and drop it in carefully, stirring meanwhile with a glass rod. A brown precipitate of Oxide of Silver first forms, but on the addition of more Ammonia it is re-dissolved.[45] When the liquid appears to be clearing up, add the Ammonia very cautiously, so as not to incur an excess. In order still further to secure the absence of free Ammonia, it is usual to direct, that when the liquid becomes perfectly clear, a drop or two of solution of Nitrate of Silver should be added until a _slight turbidity_ is again produced. Lastly, dilute with water to the proper bulk. If the crystals of Nitrate of Silver employed contain a large excess of free Nitric Acid, no precipitate will be formed on the first addition of Ammonia. The free Nitric Acid, producing _Nitrate of Ammonia_ with the alkali, keeps the Oxide of Silver in solution. This cause of error however is not likely to happen frequently, since the amount of Nitrate of Ammonia required to prevent all precipitation would be considerable. From the same reason, viz. the presence of Nitrate of Ammonia, it is often useless to attempt to convert an old Nitrate Bath already used for sensitizing, into Ammonio-Nitrate. [45] If the excess of Ammonia does not readily dissolve it, probably the Nitrate of Silver is impure. Ammonio-Nitrate of Silver should be kept in a dark place, being more prone to reduction than the Nitrate of Silver.