404 EXPERIMENTAL INVESTIGATIONS. 



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

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

 to electrisation 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 

 accurate 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 electrised a plate in the same manner, and then 

 covered the surface having the invisible image, with iodised 

 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 mariner as for a photograph. It was then held opposite 

 a window 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 the glass beyond the limits of 

 the tinfoil, were darkened, and came out with perfect distinct- 

 ness, the other parts of the glass having been, as it were, pro- 

 tected by electrisation from the action of light ; the figures 

 were permanently fixed by a strong solution of hyposulphite 

 of soda. 



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

 fixing the impression the collodion film was floated off; this 

 contained the impression, as it does with an ordinary photo- 

 graph ; and the glass plates being washed with distilled water 

 and dried, showed no impression when breathed on. 



6. An electric impression of the word ' Volta ' was well 

 rubbed with a handkerchief, then washed with water and 

 alcohol, then dried ; the impression still came out by breathing 

 upon it. Some one of the chemicals used in the collodion 

 process had probably had the effect of removing the figure in 

 Experiment 5, but I have not yet ascertained to which this 

 removal is due. 



7. Letters cut in tinfoil were substituted for those of 



