_Silver Printing._--A sheet of paper, previously treated with a solution of chloride of sodium and dried, is sensitized in an alkaline bath of nitrate of silver. When the paper is exposed under a negative, the light through the transparent parts of the negative reduces the silver, converting the chloride, it is supposed, into a metallic sub-chloride of silver which becomes dark or black, and constitutes the main portion of the picture. The image is then fixed by dissolving out the chloride of silver unaltered by light in a bath of hyposulphite of soda. After fixation, the image is well washed in several changes of water to eliminate all traces of the hyposulphite of soda and prevent the subsequent fading of the darkened portions of the picture and the yellowing of the whites. If the printed image is immediately fixed, it will have a red color. To avoid this it is washed first in water and then immersed in a chloride of gold toning bath and fixed. _The Platinotype Process_ is one in which potassium chloroplatinite and ferric oxalate are converted by light into the ferrous state, and metallic platinum is reduced when in contact with the ferrous oxalate of potash solution. The unacted upon portions are dissolved out by dilute hydrochloric acid, leaving a black permanent image. This process is characterized by simplicity, sensitiveness in action, permanence of print, and a peculiarly soft and artistic quality in the picture. British Patent No. 2,011, of 1873, to Willis, is the first disclosure of the platinotype. _Carbon Printing_ is a process in which lampblack or other indestructible pigment is mixed with the chemicals to render the photograph more stable against fading from the gradual decomposition of its elements. Mungo Ponton, in 1838, discovered the sensitive quality of potassium bichromate, which led up to carbon printing. Becquerel and Poitevin, in Paris, in 1855, were the first to experiment in this direction, and Fargier, Swan, and Johnson were successors who made valuable contributions. _Emulsions._--A photographic emulsion is a viscous liquid, such as collodion or a solution of gelatine, containing a sensitive silver salt with which the glass plate is at once coated, instead of coating the plate with collodion or gelatine, and then immersing it in a sensitizing bath. The desirability of emulsions was recognized as early as 1850 by Gustave Le Gray, and in 1853 by Gaudin. Collodion emulsion with bromide of silver was invented by Sayce and made known in 1864. In 1871 Maddox published his first notice of gelatine emulsion, and in 1873 the gelatine emulsions of Burgess were advertised for sale. In 1878 Mr. Charles Bennett brought out gelatino-bromide emulsion of extreme sensitiveness, by the application of heat, and from this time gelatine began to supersede all other organic media.