252 PROCESS OF ENGKAVING UPON SILVER. 
hyposulphite of soda, to dissolve iodide of silver which oc- 
cupies tlie place of the black parts; it is then washed with 
a large quantity of distilled water and dried. The plate is 
then heated sufficiently to oxidize the surface of the cop- 
per, which successively assumes diifereut tinis; when it 
exhibits that of a dark brown, it is allowed to cool; the 
silver exposed is amalgamated, heating the plate gently in 
order to fiicilitate the operation. As the mercury does not 
combine with the oxide of copper, we obtain an impression 
in which the amalgamated portions represent the blacks, 
and the parts of the plate covered by oxide of copper, the 
the whites ; when the amalgamation is finished, the plate is 
covered with two or three layers of gold leaf, and the 
mercury evaporated by heat ; the gold consequently ad- 
heres solely at tlie places occupied by the blacks of the en- 
graving. The gold which does not adhere is removed 
with a brush, which being done, the oxide of copper is dis- 
solved in a solution of nitrate of silver, and the silver, as 
well as the copper which is beneath it, eaten away with di- 
lute nitric acid. The lines of the drawing which are pro- 
tected by the gold not being attacked, etchings of any depth, 
corresponding to the white parts of the engraving, may be 
obtained. When tiiis last operation is done, the plate, which 
may be compared to an etched copperplate, is fit for taking 
proofs after llie matmer of wood engravings. 
To obtain plates engraved after the manner of copper- 
plates, it is requisite to operate on a gilt plate of copper. In 
the bath of sulphate of copper, tiie parts corresponding to 
the wiiites are again covered with copper ; the iodine, or the 
iodine compound wiiich was formed, is removed wiiii hypo- 
sulphite ; the layer of deposiied co[)per oxidized ; the gold 
amalgamated, which may then be removed with nitric acid, 
and at the same time the oxide of copper dissolved. In this 
way the white parts are evidently preserved, and the hol- 
lows represent the black portions, as in engraved copper- 
plates. — Chem* Gaz.. from Conijjles Rendus, 
