Sensitizing paper with Ammonio-Nitrate.--It is not usual to float the paper when, the Ammonio-Nitrate of Silver is used. If a bath of this liquid were employed, it would not only become quickly discoloured by the action of organic matter dissolved out of the papers, but would soon contain abundance of free Ammonia (see the Vocabulary, Part III., art. "Ammonio-Nitrate"); and an excess of Ammonia in the liquid produces an injurious effect by dissolving away the sensitive Chloride of Silver. The Ammonio-Nitrate is therefore applied with a glass rod, or by brushing, and in neither case is any of the liquid which has once touched the paper allowed to return into the bottle. Brushes are manufactured purposely for applying Silver solutions, but the hair is soon destroyed unless the brush be kept scrupulously clean. Lay the salted sheet upon blotting-paper, and wet it thoroughly by drawing the brush first lengthways and then across. Allow it to remain flat for a minute or so, in order that a sufficient quantity of the solution may be absorbed (you will see when it is evenly wet by looking along the surface), and then pin up by the corner in the usual way. If, on drying, white lines appear at the points last touched by the brush, it is probable that the Ammonio-Nitrate contains free Ammonia. The employment of a glass rod is a very simple and economical mode of applying Silver solutions. Procure a flat piece of board somewhat smaller than the sheet to be operated on, and having turned over the edges of the paper, secure them with a pin. Next bring the board near to the corner of the table, and laying the glass rod along the edge of the paper, allow the fluid to drop into the groove so formed; then carry the rod directly across the sheet, when an even wave of fluid will be spread over the surface. A pipette made of glass tubing, when dipped into the bottle and the upper end closed with the finger, will withdraw as much of the Ammonio-Nitrate as is required; and if a scratch be made upon the tube at a point corresponding to 30 or 40 minims, it will be found sufficient for a quarter sheet of the Papier Saxe. Ammonio-Nitrate paper, however prepared, cannot be kept many hours without becoming brown and discoloured.