64 Mr. W. R. Grove on some neio Methods of 



chief until their surfaces gave a uniforrs flush when breathed 

 on. Between these plates was then placed a piece of hand-bill 

 printed on one side only ; pieces of tinfoil rather smaller than 

 the glasses were placed on the outside of each, and these coatings 

 were connected with the secondary terminals of a Ruhmkorff 

 coil. After a few minutes' electrization, the coatings were care- 

 fully removed, and the interior surface of the glass when breathed 

 on, showed with great beauty the printed words which had been 

 opposite it, these appearing as though etched on the glass, or 

 having a frosted appearance ; even the fibres of the paper were 

 beautifully brought out by the breath, but nothing beyond the 

 margin of the tinfoil. 



2. It now occurred to me that I might render these impres- 

 sions permanent by the use of hydrofluoric acid, and a similar 

 experiment was made, the naked plate of glass after electrization 

 being exposed over a leaden dish containing powdered fluor spar 

 and sulphuric acid, and slightly warmed; the letters came out 

 rather imperfectly, but some creases in the paper were beauti- 

 fully reproduced. 



3. 1 now cut out of thin white letter-paper the word Volta, 

 and placed it between the plates of glass ; they were submitted 

 to electrization as before, and the interior surface of one of 

 them, without the paper letters, was subsequently exposed to 

 the hydrofluoric acid vapour; the previously invisible figures 

 came out perfectly, and formed a permanent and perfectly accu- 

 I'ate etching of the word Volta, as complete as if it had been 

 done in the usual mode by an etching ground. This of course 

 could be washed and rubbed to any extent without alteration, 

 and the results I have obtained give every promise for those who 

 may pursue this as an art, of producing very beautiful effects, 

 enabling Silhouette designs, or even fine engravings, to be copied 

 on glass, &c. 



4. I again electrized a plate in the same manner, and then 

 covered the surface having the invisible image, with iodized 

 collodion, and immersed it in a bath of nitrate of silver (40 

 grains to the ounce) in a room lighted by a candle, in the usual 

 manner as for a photograph. It was then held opposite a win- 

 dow for a few seconds, and taken back into the darkened room, 

 and on pouring over it a solution of pyrogallic acid, the word 

 Volta, and the border of theg lass beyond the limits of the tin- 

 foil, were darkened, and came out with perfect distinctness, the 

 other parts of the glass having been, as it were, protected by 

 electrization from the action of light ; the figures were perma- 

 nently fixed by a strong solution of hyposulphate of soda. 



5. A similar experiment to the last was made, but after fixing 

 the impression, the collodion film was floated ofi^; this contained 



