Proceedings of the British Association. 275 



1 On Fizeau's Process of Etching Daguerreotype Plates, and its 

 Application to Objects of Natural History/ by Mr. Goadby. — In a 

 Daguerreotype portrait, the black parts of the plate consist of silver, 

 the white of mercury, and the intermediate tint of a mixture of the 

 two, the degree of darkness or light depending upon the excess either 

 of the silver or of the mercury. In converting a Daguerreotype into 

 an engraved plate, it is necessary to etch away the dark parts and to 

 leave the white untouched. This is done by immersing the plate in 

 a fluid, consisting of dilute nitric acid, nitrous acid, chloride of 

 sodium, and nitrate of potash. The nitric acid is so far diluted, that 

 no decomposition can take place until the mixture is heated, when 

 the chloride of sodium and nitrate of potash are decomposed, and 

 chlorine and nitrous acid are evolved. These attack and remove the 

 silver, or the dark portions of the plate, but have no effect on the 

 mercury, so that the lights of the picture, being the mercurialized 

 portions of the plate, constitute the etching ground, and effectually 

 defend such portions of the Daguerreotype from the influence of the 

 corroding fluid. After a time, those portions of the plate that have 

 been acted upon by the chlorine, &c. become covered with a protect- 

 ing coat of the chloride of silver : this must be removed by dilute 

 liquid ammonia, when the biting may be continued by a fresh supply 

 of the mixed acid. Grease and foreign matter must be removed by 

 repeated washings in dilute acid and alkali, and by boiling in caustic 

 potash. These cleansing operations must be repeated after every 

 biting, after washing out the chloride of silver by the ammonia. The 

 plate being thus bitten, but in a slight degree, is to be inked after 

 the ordinary manner of engravers, and allowed to dry ; the surface 

 of the plate is then to be thoroughly polished, the ink still remaining 

 in the corroded portions of the plate. It is now to be gilded by the 

 electrotype, those parts alone receiving the gold that have been pre- 

 viously polished. The ink is then to be dissolved out of the hollows 

 by potash : the parts that are gilded now constitute the etching 

 ground, instead of the mercury, and the biting may be henceforth 

 continued by nitric acid, in the customary usage of engravers. The 

 plate thus etched generally requires to be finished by the hand of the 

 engraver, who has the advantage of a perfect, although faint picture 

 to work upon. The amount of labour which he must bestow will dc- 



