MECHANICS AND USEFUL ARTS. 87 



ON GALVANIC DEPOSITION OF IRON IN COHESIVE FILMS. 



A German chemist of eminence, Dr. F. Varrentrapp, of Bruns- 

 wick, comes to the conclusion that there are no reasons why de- 

 posits of almost any thickness of metal in cohesive coats may not 

 be obtained from solutions of iron by moans of the galvanic cur- 

 rent, with as great a facility as from those of copper, if only cer- 

 tain jDrecautionary rules be attended to, which follow : — 



From a watery solution of iron in which a plate of metallic iron, 

 serving as, and connected with, the copper cylinder of a simple 

 Daniell element, and the plate to receive the deposit connected 

 with the zinc cylinder are immersed, both plates being of about 

 the same size, a coating, though thin, will be obtained, but it will 

 remain so, and usually there will very soon set in a strong evolu- 

 tion of gas. In order to avoid this, it is sufficient to insert in the 

 current close to the iron plate a spiral of iron wire, thus enlarging 

 the surface of this metal as compared to that of the one which i3 

 to receive the coating, and upon this change the process will go 

 on for days in perfect regularity. The iron coating has the ten- 

 dency of forming warty aggregations near the edges. It is ex- 

 ceedingly brittle, and of a hardness resembling non-hardened steel 

 under the file. After passing the fire, however, it becomes soft 

 and flexible, and may be rolled into a narrow cylinder. 



The tank must be of such dimensions that there may remain a 

 space of at least 4 to 5 inches between both plates. It is of advan- 

 tage to arrange in front of the iron plate a pane of glass, not 

 quite reaching to the sides of the vessel, as this will prevent the 

 particles from being carried on to the surface of the plate. 



Metallic types and blocks are most readily coated in this 

 manner, as much so as with deposits of copper; they may be 

 easily separated by having a previous coating of silver, which has 

 been allowed to become yellow in an atmosphere of sulphuretted 

 hydrogen, but which must not be too heavy, since in that case it 

 would cause the iron coating to become loose and reflexed, as 

 soon as it acquires about the thickness of paper. The latter also 

 takes place on casts of wax or gutta-percha made conductive by 

 means of jolumbago, in case the coating cannot be made to grow 

 beyond the edges. Any bubbles of air which may adhere to the 

 object will prevent a correct deposition, and must be removed by 

 immersing in alcohol previous to placing in position, or by blow- 

 ing over it atomized spirits. 



iSince the iron solution is rarely quite neutral, and generally to 

 some extent oxidized, it is w^ell to receive the first deposit on a 

 spare plate of copper, to remain in the bath for an hour. When 

 the battery is fresh, or must be renewed, it is well also to add some 

 sulphuric acid to the strong solution of copper. The water for the 

 amalgamated zinc should contain only short of 3 per cent, of sul- 

 phuric acid. 



In the beginning of the operation the formation of bubbles 

 cannot always be avoided altogether, and it will be found best to 

 remove the plate or object after 5 minutes of action, to rinse it in 



