ETCHING DAGUERREOTYPES. 243 



2. The intensity of the voltaic current. Here it appeared 

 to me that, as in the electrotype, where the visible action is 

 at the cathode, a certain degree of intensity throws down 

 metal as a crystal, an increased intensity as a metallic plate, 

 and a further intensity as a pulverulent mass ; that degree of 

 intensity which would show on the negative deposit the finest 

 impressions from the cathode, would also produce on the 

 anode the most delicate excavations, and consequently, an 

 intensity which would just fall short of the point of evolving 

 oxygen from the plate to be etched, would be the most likely 

 to succeed ; this point was not, however, adopted without care- 

 ful experiment, the more so, as in one instance Mr. Gassiot 

 succeeded in procuring a very fair etching with a series of 

 ten pairs of the nitric acid battery ; however, the results of 

 repeated experiments, in which the intensity has been varied 

 from a series of sixteen pairs to one of the nitric acid battery, 

 were strongly in favour of the above idea, and consequently 

 went to prove that one pair gives the most efficient degree of 

 intensity for the purpose required. 



3. The distance between the plates. As it was proved 

 by De la Rive that in an electrolytic solution, when the 

 electrodes are at a distance, the action extends a little beyond 

 the parallel lines which would join the bounds of the elec- 

 trodes, and thus, that the current, as it were, diverges and 

 converges, it appeared advisable to approximate the electrodes 

 as nearly as possible, so as to produce uniformity of action 

 over the whole plate. Provided a solution be used which 

 does not evolve gas at the cathode, I am inclined to think 

 that the plates may be with advantage indefinitely approxi- 

 mated ; but as this was not the case with the solution I 

 selected for the greater number of experiments, 0*2 of an 

 inch was fixed on as the distance, in order that the gas 

 evolved from the cathode should not adhere to the anode, 

 and thus interfere with the action. 



4. Time of continuing the operation. This was a matter 

 only to be decided by experiment, and must vary for the vol- 

 taic combination arid solution employed. With a single pair 

 of the nitric acid battery, from twenty-five to thirty seconds 

 was, after a great number of experiments, fixed on as the 



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