DAGUERREOTYPE PORTRAITS FROM THE LIFE FIRST TAKEN. 



(From the London and Edinburgh PltUotophical Magazine for September, 1840.) 



CONTENTS : Daguerreotype Portraits from the Life first taken. Spectral Images. 

 Preservation of the Sensitive Plate increases the Sensitiveness. Modifications in the 

 Daguerreotype Process. Moonlight, Artificial Light, and Drummond's Light, are 

 all active. Description of the original Process of taking Portraits from the Life. 



523. VERY soon after M. DAGUERRE'S remarkable process for photogenic drawing was 

 known in America, I made attempts to accomplish its application to the execution of 

 portraits from the life. M. ARAGO had already stated, in his address to the Chamber 

 of Deputies, that M. DAGUERRE expected, by a slight advance, to meet with success, but 

 as yet no account has reached us of that object being attained. 



524. More than one hundred instances are recorded in BERZEI.IUS'S chemistry, in 

 which the agency of light brings about changes in bodies ; these are of all kinds : for- 

 mations of new compounds, rearrangements of elements already in union, changes of 

 crystallographic character, decompositions, and mechanical modifications. 



525. The process of the Daguerreotype is to expose a surface of pure silver to the 

 action of the vapour of iodine, so as to give rise to a peculiar iodide of silver, which, 

 under certain circumstances, is exceedingly sensitive to light. The different operations 

 of polishing, washing with nitric acid, exposure to heat, &c., are only to offer a pure 

 silver surface ; the operation of hyposulphite of soda, and the process, which I shall 

 presently describe, of galvanization, are to free the plate from its sensitive coating, and 

 in nowise affect the depth of the shadows, as some of the French chemists at first 

 supposed. 



526. There is but one part of the Daguerreotype which does not yield to theory : 

 on one point alone there is obscurity. Why does the vapour of mercury condense in 

 a white form on those portions of the film of iodide which have been exposed to the 

 influence of light ? condense to an amount which is rigidly proportional to the quantity 

 of incident light I 



527. Even on this point there are facts which appear to have a bearing 



(.) It has long been known that if a piece of soapstone or agalmatolite be made use 

 of as a pencil to write with on glass, though the letters that may have been formed are 

 invisible, and though the surface of the glass may subsequently have been well cleaned, 

 yet they will come into view as soon as the glass is breathed on. 



(5.) I have often noticed that if a piece of very clear and cool glass, or, what is bet- 

 ter, 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 carefully removed, as 

 often as you breathe again on the surface, a spectral image of it may be seen, and this 

 singular phenomenon 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. 



In (491), (521) I have described a phenomenon which relates to the crystalli- 



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