There are two modes of iodizing and sensitizing the sheets: first, by floating alternately upon Iodide of Potassium and Nitrate of Silver, in the same manner as in the preparation of papers for Positive Printing; and second, by what is termed "the single wash," which is thought by many to give superior results as regards sensitiveness and intensity of image. To iodize by this mode, the yellow Iodide of Silver, prepared by mixing solutions of Iodide of Potassium and Nitrate of Silver, is dissolved in a strong solution of Iodide of Potassium; the sheets are floated for an instant upon this liquid and dried; they are then removed to a dish of water, by the action of which the Iodide of Silver is precipitated upon the surface of the paper in a finely divided state. The properties of a solution of Iodide of Silver in Iodide of Potassium, or of the double Iodide of Potassium and Silver, are described at page 43, a reference to which will show that the double salt is decomposed by a large quantity of water, with precipitation of the Iodide of Silver, this substance being insoluble in a dilute solution of Iodide of Potassium, although soluble in a strong solution. Paper coated with Iodide of Silver by this mode, after proper washing in water to remove soluble salts (which if allowed to remain would attract damp), will keep good for a long time. The layer of Iodide appears of a pale primrose colour, and is perfectly insensitive to light. Even exposure to the sun's rays produces no change, thus indicating that an excess of Nitrate of Silver is essential to the visible darkening of Iodide of Silver by light. The paper is also insensitive to the reception of an invisible image, differing in this respect from the washed Collodion plate, which receives an impression in the Camera, although apparently freed from Nitrate of Silver. To render Calotype paper sensitive to light, it is brushed with a solution of Nitrate of Silver containing both Acetic and Gallic Acids, termed "Aceto-Nitrate" and "Gallo-Nitrate" solution. The Gallic Acid lessens the keeping qualities of the paper, but increases the sensitiveness. The Acetic Acid prevents the paper from blackening all over during the development, and preserves the clearness of the white parts; its employment is indispensable. The paper is commonly excited upon the morning of the day upon which it is intended to be used; and the longer it is kept, the less active and certain it becomes. An exposure of five to eight minutes in the Camera is the average time with an ordinary view lens. The picture is developed with a saturated solution of Gallic Acid, to which a portion of Aceto-Nitrate of Silver is added to heighten the intensity. Both Sulphate of Iron, and Pyrogallic Acid have also been used, but they are unnecessarily strong, the invisible image being more easily developed upon paper than upon Collodion (see page 143).