ELECTRO-METALLURGY 215 



electrotype long before the art was known. Brass pins are thrown into solution of tin 

 in cream of tartar, and are unchanged ; but when a lump of tin is thrown among them, 

 a voltaic pair is formed, and tin is deposited on all the heap. Any stray pins detached 

 from the mass, escape the influence. 



Space would fail us were we to go through the list of crystalline and of simplo 

 bodies formed by these processes ; as for instance, octahedral crystals of protoxide of 

 copper ; tetrahedral crystals of proto-chloride of copper ; octahedral crystals of sul- 

 phide of silver ; crystals of subnitrate of copper ; bibasic carbonate of copper, and 

 others too numerous to name, have all been formed by slow voltaic actions. The alka- 

 line metals, potassium, sodium, &c., were first obtained by Davy in the galvanic way; 

 magnesium, barium, aluminium, calcium, &c., are obtained by M. Bunsen by operating 

 upon the chlorides of these metals either in solution or in a state of fusion. 



Electro-etching is produced at the place where the current enters the decomposing 

 trough, as at the copper-plates c of fig. 794. A plate of copper is prepared as if for 

 the graver ; its face is then covered with an etching ground of asphalte, wax, black 

 pitch, and burgundy pitch ; and its back with varnish. The design is then traced 

 through the etching ground with a fine point ; the plate is then placed in the trough 

 B, containing either sulphate of copper or simply diluted sulphuric acid, and connected 

 with the copper of the battery. After a few minutes it is removed, and the fine lines 

 are stopped out with varnish ; it is then replaced, and again, after a few minutes is 

 removed, and the darker shades are stopped out ; the parts still exposed are again 

 subjected to the action, and the etching is complete. When the ground is removed, 

 the design will be found etched upon the copper-plate ready for the printer. 



Daguerreotype etching is a delicate operation, and requires much care. The solution 

 employed by Professor (now Sir "VV.) Grove was hydrochloric acid and water in equal 

 parts, and a battery of two or three cells. 



Platinized silver is used in face of the daguerreotype, instead of copper. The result 

 comes out in about half a minute. An oxy-chloride of silver is formed, and the mer- 

 cury of the plate remains untouched. 



A Photo-galvano-grapkic Company was formed in London for carrying out the 

 process of Paul Pretsch. He made solutions of bichromate of potash in glue-water, or 

 in solution of gelatine, instead of in pure water. He then treated the glass or plate 

 with these, and in the usual way took a picture. He washed the gelatine picture with 

 water, or solution of borax or carbonate of soda, which left the picture in relief ; when 

 developed, he washed with spirits of wine, and obtained a sunk design. The surfaces 

 thus prepared, or moulds made from them, were placed in a galvano-plastic apparatus 

 for obtaining an engraved plate from which to print. The process was not, commer- 

 cially, successful. See PHOTO-GALVANOGTBAPHY. 



The Duke of Leuchtenberg prepares a plate for etching by leaving the design on 

 the ground, and removing the ground for the blank parts. When his electrotype 

 operation is complete, the design is in relief instead of being in intaglio as in ordinary 

 etching. 



Mctallo-chromes consist of thin films of oxide of lead, deposited sometimes on 

 polished plates of platinum, but most commonly on polished steel plates. The colours 

 are most brilliant and varied. Nobili is the author of the process. 



A saturated solution of acetate of lead is prepared and placed in a horizontal trough. 

 Three or four battery cells are required. A steel plate is laid in the acetate of lead 

 with its polished surface upward, and is connected with the copper of the battery. If 

 a wire is connected with the zinc end of the battery, and held over the steel plate in 

 the solution, a series of circles in brilliant colours, arise from the spot immediately 

 beneath the wire, and expand and spread, like the circles when a stone is thrown into 

 a pond. Silver-blond is the first colour ; then fawn-colour, followed by the various 

 shades of violet, and indigoes and blues ; lake, bluish lake, green and orange, greenish 

 violet, and passing through reddish yellow to rose-lake, which is the last colour in the 

 series. 



According to the shape of the metal by which the current enters be it a point, 

 a slip, a cross, a concave, or a convex disc so is the form of the coloured figure 

 varied. And if, in addition to this, a pattern in card or gutta-percha is cut out and 

 interposed between the two surfaces, the action is intercepted by the portions not 

 removed, and the design is produced on the steel plate, in colours, that may be greatly 

 varied, according to the duration of the experiment. The different colours are due to 

 the different thicknesses of the thin films of peroxide of lead. 



M. Becquerel proposed the deposit of peroxide of lead, and also the red peroxide of 

 iron, for protecting metals from the action of the atmosphere. For the latter, proto- 

 sulphate of iron is dissolved in ammonia solution, and operated upon by two or three 

 batteries. 



The most important application of electro-metallurgy in the arts has been for 



