236 
MISCELLANY. 
the mercury. In converting a Daguerreotype into an engraved plate, 
it is necessary to etch away the dark parts and to leave the white un- 
touched. This is done by immersing the plate in a fluid, consisting of 
dilute nitric acid, nitrous acid, chloride of sodium, and nitrate of pot- 
ash. 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 upon 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 Daguer- 
reotype 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 protecting 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. 
Tt is now to be gilded by the electrotype, those parts alone receiving 
the gold that have been previously 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, w T ho has the advantage of a perfect, 
although faint picture to work upon. The amount of labour which he 
must bestow will depend upon the goodness of the Daguerreotype, and 
the success of the etching. M. Claudet has fully established the suc- 
cessful application of this process to the purposes of illustrating Natu- 
ral History, by copying from Nature and engraving several delicate 
and difficult dissections of the lower animals, particularly the nervous 
system of Aplysia and Tritonia (the latter much magnified.) and the 
nutrimental organs in situ of a caterpillar. These preparations 5 to- 
gether with the engravings of them, were handed round. 
Dr. Carpenter stated, that a similar process had been employed for 
engraving microscopic objects, the discovery of which was due to 
Capt. Ibbetson. He exhibited some plates of blood-globules, and 
