But independent of these differences, the chemical nature of the _size_ employed also influences the toning of the Print. For instance, in the process above given, if the Positives, after having been fully toned in the Gold Bath, and washed in cold water, be treated with _boiling water_, the tint immediately changes to a dull red; but on blotting off between sheets of bibulous paper and pressing with a hot iron, the dark tones are restored. This destruction of the tint by boiling water, and its restoration by _dry heat_, is due in great part to the animal substance employed in sizing the paper; and it will be found that prints upon a foreign paper, such as the Saxony Positive, salted with a ten-grain solution and sensitized with Ammonio-Nitrate, do not lose their tones in hot water and are not much darkened by ironing. The peculiarity of the sizing of the English Photographic papers must therefore be borne in mind, and allowance made for the additional sensitiveness and alteration of colour which it produces. When a formula is given, the paper which is recommended for that particular formula should alone be used. SECTION II. _Positive Printing by Development._ Negative printing processes will be found useful during the dull winter months, and at other times when the light is feeble, or when it is required to produce a large number of impressions from a Negative in a short space of time. The plan of development also enables the operator to obtain Positives of greater stability than those yielded by the direct action of light. Three processes may be described, the first of which gives Positives of an agreeable colour, but the second, on Iodide of Silver, the greatest permanency under unfavourable conditions. NEGATIVE PRINTING PROCESSES UPON CHLORIDE OF SILVER. Positives may be obtained by exposing paper prepared with Chloride of Silver to the action of light until a faint image is perceptible, and subsequently developing by Gallic Acid; but in this process it is difficult to obtain sufficient _contrast_ of light and shade; the impression, if sufficiently exposed and not too much developed, being feeble, with a want of intensity in the dark parts. By associating with the Chloride an organic salt of Silver, such as the Citrate, this difficulty may be overcome, and the shadows be brought out with great depth and distinctness. The papers are salted with a mixed Chloride and Citrate as in the formula for the Ammonio-Nitrate Process.[49] They are then rendered sensitive upon a Bath of Nitrate of Silver _containing_ either Citric or _Acetic Acid_, which are used in Negative processes to preserve the clearness of the white parts under the influence of the developer. [49] The formula at p. 246 may be modified with advantage: use double the quantity of Gelatine, and half the amount of Citrate and Chloride. The Bath of Aceto-Nitrate is prepared as follows:--