HISTORY OF PHOTOGRAPHY. 191 



When the glass is coated with the bromide of silver, the action per se, is very 

 slow, and the discoloration ultimately produced far short of blackness ; but 

 when moistened with nitrate of silver, sp. gr. 1-1, it is still more rapid than 

 with the iodide, turning quite black in the course of a very few seconds' ex- 

 posure to sunshine. Plates of glass thus coated may be easily preserved for 

 the use of the camera, and have the advantage of being ready at a moment's 

 notice, requiring nothing but a wash over with the nitrate of silver, which may 

 be delayed until the image is actually thrown on the plate and adjusted to the 

 connect focus with all deliberation. The sensitive wash being then applied 

 with a soft, flat camel's-hair brush, the box may be closed and the picture im- 

 pressed; after which it only requires to be thrown into water and dried in the 

 dark to be rendered comparatively insensible, and may be finally fixed with 

 hyposulphite of soda, which must be applied hot, its solvent power on the 

 bromide being even less than on the iodide. 



Sir John Herschel suggested a trial of the fluoride of silver upon 

 glass, which, he says, if proved to be decomposable by light, might 

 possibly effect an etching on the glass by the corroding property of 

 the hydrofluoric acid. 



The metallic fluorides have been found to be decomposable, and a 

 very sensitive process on paper, called the fluorotype, will be de- 

 scribed in the chapter on JNliscellaneous Processes. I am not aware 

 that any experiments have been made directly upon glass, but it is 

 certainly worthy of a careful trial. 



Herschel has remarked that we can not allow the wash of nitrate 

 to dry upon the coating of the chloride or iodide of silver. If,. 

 hoAvever, we dip a glass which has one film of chloride upon it into 

 a solution of common salt, and then spread upon it some nitrate of 

 silver, we may very materially thicken the coating, and thus produce 

 more intense effects. Mr. Towson employed glass plates prepared 

 in this manner with much success. The mode adopted by that 

 gentleman was to have a box the exact size of the glass plate, in the 

 bottom of which was a small hole ; the glass was placed over the bot- 

 tom, and the mixed solution, just strong enaugh to be milk}^ of the 

 salt and silver poured in. As the fluid finds its way slowly around the 

 edges of the glass, it filters out; the peculiar surface action of the 

 solid glass plate, probably a modified form of cohesive force, sepa- 

 rating the fine precipitate which is left behind on the surface of 

 the plate. By this means the operation of coating the glass is much 

 quickened. Another method by which films of any of the salts of sil- 

 ver can be produced upon glass plates, is the following modification of 

 the patent processes of Drayton and of Thompson for silvering glass : 

 Take a ver}^ clear plate of glass, and having put around it an edging 

 of wax about half an inch in depth, pour into it a solution of nitrate 

 of silver made alkaline by a few drops of ammonia, taking care that 

 no oxide of silver is precipitated; mix with this a small quantity of 

 spirits of wine, and then add a mixture of the oils of lavender and 

 cassia, or, which is perhaps the best process, a solution of grape sugar. 



