184 ANNUAL OF SCIENTIFIC DISCOVERY. 



tempt to enter the lines into the base metal, the result will be extremely ir- 

 regular and partial. Again, if the film of silver is not quite sufficient in 

 thickness, the lights of the Daguerreotype will be attacked, and the shades 

 blurred by the spread of the lines. The problem depends, therefore, on get- 

 ting a good protecting film, and then a good biting through this film. If the 

 thickness of the film was not, unfortunately, so nearly equal to the depth to 

 which the silver can be etched, I think the making of an engraved plate by 

 chemical means would be comparatively easy. 



" A copper plate should have the surface prepared very perfectly. It is 

 then to be electro-plated to the weight of one eighth of a grain to the square 

 inch ; but this thickness is not } r et well determined. The plate, after being 

 washed in distilled water and dried, is to be slightly buffed, and the Daguerre- 

 otype taken in the usual manner. Professor Grove recommended chlorohydric 

 acid as the electrolyte for etching ; but, in acting on a plate of base metal merely 

 enfilmed with the silver, we evidently should select an electrolyte which will 

 not be liable to act on the base metal. Of the various electrolytes I tried, I 

 found chloride of sodium the best: this seems to free the generated chloride 

 of silver more readily than clorohydric acid, is without much action on the 

 copper base, and has the very great advantage of being free from poisonous 

 qualities or disagreeable exhalations. 



" The object of the biting process, so far, has been only to remove the silver 

 from the parts where we want the copper to be bitten in deep lines ; but this 

 process evidently can not be continued till the copper has been bitten suffi- 

 ciently deep, for it acts on the silver as well as the copper ; hence, the first 

 biting should be continued only long enough to work through the silver film ; 

 here there is a liability of spoiling the work in the beginning, by overdoing 

 the first biting, which will infallibly remove the silver from the lights in some 

 places, and give the whole plate a blurred or mezzotinted appearance. I can 

 give no directions for the time of the biting through the film ; practice and 

 dexterous manipulation, as in the Daguerreotype process, are the only helps 

 here. After the plate has been bitten through the film, it should be washed 

 by immersing in water, and dried over a current of heated air. If. on exami- 

 nation, there appear no marked defects, the process of entering the lines into 

 the copper may be gone on with. For this purpose I have used perchloride 

 of iron, persulphate of iron, and also nitrate of silver. I have not determined 

 which of these is the best ; but, so far, I have a preference for the perchloride 

 of iron. The perchloride may be added to water till it has a lemon-yellow 

 color. The plate is to be immersed in a horizontal position, with the face up 

 in the solvent, and a soft camel's-hair pencil swept gently over it from time to 

 time. In the course of thirty minutes, or less, the action of the perchloride 

 will have thrown up the chloride of silver, so that the brush can sweep it 

 away, and the bright copper will appear in the bottoms of the lines. The 

 plate may then be washed and dried, and if, on inspection, it should not be 

 thought deep enough to hold the ink for printing, it may be returned to the 

 bath of perchloride for a short time. I have not yet obtained a plate which 

 has not been much corroded in the lights ; and this corroding in the light has 

 caused rne to discontinue the second biting, before the depths of the lines has 



