and on the recent discovery of Moser. 467 



of copper which may be formed ; when quite dry a little mer- 

 cury taken up on soft leather or linen is well rubbed over it, 

 and the surface worked to a perfect mirror. 



The sheet to be copied is placed smoothly over the mercu- 

 rial surface, and a sheet or two of soft, clean paper being 

 placed upon it, it is pressed into equal contact with the metal 

 by a piece of glass, or flat board ; in this state it is allowed to 

 remain for an hour or two. The time may be considerably 

 shortened by applying a very gentle heat for a few minutes to 

 the under surface of the plate. The heat must on no account 

 be so great as to volatilize the mercury. The next process 

 is to place the plate of metal in a closed box, prepared for 

 generating the vapour of mercury. The vapour is to be slow- 

 ly evolved, and in a few seconds the picture will begin to ap- 

 pear ; the vapour of mercury attacks those parts which corre- 

 spond to the white parts of the printed page or engraving, and 

 gives a very faithful, but a somewhat indistinct image. The 

 plate is now removed from the mercurial box, and placed into 

 one containing iodine, to the vapour of which it is exposed for 

 a short time; it will soon be very evident that the iodine va- 

 pour attacks those parts which are free from mercurial vapour, 

 blackening them. Hence there results a perfectly black pic- 

 ture, contrasted with the gray ground formed by the mercu- 

 rial vapour. The picture being formed by the vapours of 

 mercury and iodine, is of course in the same state as a Da- 

 guerreotype picture, and is readily destroyed by rubbing. 

 From the depth to which I find the impression made into the 

 metal, I confidently hope to be enabled to give to these sin- 

 gular and beautiful productions a considerable degree of per- 

 manence, so that they may be used by engravers for working on. 



It is a curious fact that the vapours of mercury and of io- 

 dine attack the plate differently, and I believe it will be found 

 that vapours have some distinct relation to the chemical or 

 thermo-electrical state of the bodies upon which they are re- 

 ceived. Moser has observed this, and attributes the pheno- 

 mena to the colours of the rays, which he supposes to become 

 latent in the vapour on its passing from the solid into the more 

 subtile form. I do not however think this explanation will 

 agree with the results of experiments. I feel convinced that 

 we have to deal with some thermic influence, and that it will 

 eventually be found that some purely calorific excitement 

 produces a molecular change, or that a thermo-electric action 

 is induced, which effects some change in the polarities of the 

 ultimate atoms of the solid. 



These are matters which can only be decided by a series of 

 well-conducted experiments, and, although the subject will 



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