14 HISTORY OF ELECTRO-METALLURGY. 



wax, common resin, and a small portion of plaster of Paris. This 

 seems to answer the purpose tolerably, though I have no doubt, by 

 an extended practice, a better may still be obtained by a person 

 practically acquainted with the etching-grounds in use among 

 engravers. 



" I now proceed to the second, and I believe the most satisfactory 

 portion of the subject. Although I have placed these experiments 

 last, some of them were made at the same time with the others 

 already described, and some of them before ; but, to render the sub- 

 ject more intelligible, I have placed them thus. 



" The members of the society will recollect that, on the first 

 evening it met, I read a paper on the ' production of metallic veins 

 in the crust of the earth,' and that among other specimens of cupre- 

 ous crystallization which I produced on that occasion, I exhibited 

 three coins one wholly covered with metallic crystals, the other on 

 one side only. It was used under the following circumstances. 

 When about to make the experiment, I had not a slip of copper at 

 hand to form the negative end of my arrangement, and, as a good 

 substitute, I took a penny and fastened it to one end of the wire, and 

 put it, in connection with a piece of zinc, in the apparatus already 

 described. 



" Voltaic action took place, and the copper coin became covered 

 with a deposition of copper in a crystalline form. But, when about 

 to make another experiment, and being desirous of using the piece 

 of wire used in the first instance, I pulled it off the coin to which it 

 was attached. In doing this, a piece of the deposited copper came 

 off with it ; on examining the under portion of which, I found it 

 contained an exact mould of a part of the head and letters of the 

 coin, as smooth and sharp in every respect, as the original on which 

 it was deposited. I was much struck with this at the time ; but, on 

 examination, the deposition metal was very brittle. This, and the 

 fact that it would require a metallic nucleus to aggregate on, made 

 me apprehensive that its future usefulness would be materially 

 abridged ; but it was reserved for future experiment, and hi conse- 

 quence laid aside for a time, until my attention was recalled to the 

 subject in a subsequent experiment, already detailed, by the drops 

 of varnish on a slip of copper. Finding in that instance that the 

 deposit would take the direction of any non-conducting material, and 

 be, as it were, guided by it, I was induced to give the previous 

 branch of the subject a second trial, because I had in the first instance 

 supposed that the deposition would only take place continuously, and 

 not on isolated specks of a metallic surface, as I now found it would ; 

 but the principal inducement to investigate the subject was the fact 

 of finding that deposited copper had much more tenacity than I at 

 first imagined. 



