PHOTOGRAPHY. 85 



sponcling picture, by converting the parts impressed into iodide. 

 The plate, thus prepared, is galvanized in a saturated solution 

 of sulphate of copper, connected by a strip of platinum with 

 the positive pole of a battery of several pairs. In this way, 

 the white portions of the picture are metallized, while the 

 iodized or black limnings remain untouched. The immersion 

 of the plate in the copper solution should only continue some 

 minutes ; otherwise the whole of it may become coated. 



After careful washing, solution of hyposulphite of soda is 

 applied to dissolve out the iodide of silver, covering the black 

 parts, and the plate is then heated until its surface assumes a 

 dark-brown shade. This oxidation of the copper protects the 

 white portions of the picture, which it covers, from the action 

 of the mercury next used to amalgamate the exposed silver. 

 The amalgamated plate is overlaid with several layers of gold- 

 foil, and then heated to volatilize the mercury. The lines ori- 

 ginally engraved by the iodine are thus gilded, and, after the 

 loose particles of foil are brushed off, the oxide of copper is 

 removed by a solution of nitrate of silver, and both the cop- 

 per and silver, beneath, dissolved out by nitric, acid. As the 

 gilded portions are protected, the etching may be managed to 

 any depth. The plate thus engraved furnishes impressions in 

 the same manner as wood-cuts. 



The plate must be gilt instead of silvered, if it is to be en- 

 graved after the manner of copper-plates. The process is the 

 same as above, excepting the omission of overlaying with 

 gold-leaf, until the application of the nitric acid, which serves, 

 instead of heat, to remove the amalgam, and simultaneously, 

 also, the oxide of copper. In this kind of plate, however, 

 the depressed portions furnish the black part of the proof. 



5. Photography, the art of obtaining representations of 

 objects upon surfaces rendered sensitive to the action of light, 

 is already a beautiful art, although but in its infancy. Very 

 correct representations of animate and inanimate objects, if 

 at rest, are taken upon a polished surface of silver, upon paper, 

 and lately upon glass. Some of the salts of silver, as iodide 

 and bromide, are usually employed to render the surface senai- 

 H 



