218 Professor Draper on the Process of Daguerreotype, 



agalmatolite be made use of as a pencil to write with on 

 glass, though the letters that may have been formed are invi- 

 sible, and though the surface of the glass may subsequently 

 have been well cleared, yet they will come into view as soon 

 as the glass is breathed on. 



(b,) I have often noticed, that if a piece of very clear and 

 cool glass, or what is better, a cold polished metallic reflector, 

 has a little object, such as a piece of metal, laid upon it, and 

 the surface be breathed over once, the object being then care- 

 fully removed, as often as you breathe again on the surface, 

 a spectral image of it may be seen, and this singular phaeno- 

 menon may be exhibited for many days after the first trial 

 was made. 



(c.) Again, in the common experiment of engraving on 

 glass by hydrofluoric acid, if the vapour has been very weak, 

 no traces will be perceived on the glass after the wax has been 

 removed ; but on breathing over it, the moisture condenses 

 in such a way, as to bring all the object into view. 



(d.) In a former number of this Journal I described a phe- 

 nomenon which relates to the crystallization of camphor on 

 surfaces of dry glass, on which moveable traces have been made 

 by the pressure of a glass rod ; this also appears to belong to 

 the same class of effects. 



Berzelius (Traite, vol. ii. p. 186.) has attempted to explain 

 (a,) and (c.) on this principle, that the changed and unchanged 

 surfaces radiate heat unequally. There may be strong doubts 

 with some as to the correctness of this, but is not the Daguer- 

 reotype due to the same cause, whatever it may be? 



We must separate carefully the chemical changes which 

 iodide of silver undergoes in the sunbeam, from the mechanical 

 changes which happen to the sensitive film : iodide of silver 

 turns black in the solar ray, the whole success of the Daguer- 

 reotype artist depends on his checking the process before 

 that change shall have supervened. 



The coating of iodine is not immediately necessary to the 

 production of images by the mercurial vapour. The con- 

 dition seems to be traceable to the metallic surface. If you 

 take a Daguerreotype, clean off the mercury, polish the plate 

 thoroughly with rottenstone, wash it with nitric acid and 

 bring it to a brilliant surface, yet if it has not been exposed to 

 heat, the original picture will re-appear on exposure to the 

 mercurial vapour. Is not this a result of the same kind as 

 those just referred to ? 



As a polishing material for the Daguerreotype plate, com- 

 mon rottenstone and oil answer very well. The plate having 

 been planished by the workman, is to be rubbed down to a 

 good surface, and as high a polish given to it as possible ; 



