Professor Draper on the Process of Daguerreotyp e. 217 



action of my battery. I mean, as soon as I have time to add 

 about 850 pairs of cylinders to it. Woe then to the unfor- 

 tunate wretch who comes between the poles, when connected 

 with the electrical battery ! 



P. S. Please to observe, that when I procured the stream of 

 electricity in the interval between the platina wires, I used 

 the water battery alone, without other apparatus, and not 

 connected with the electrical or any other battery. 



Broomfield, near Taunton, July 17, 1840. 



XXXII.O/i the Process of Daguerreotype, a fid its application 

 to taking Portraits from the Life. By John William 

 Draper, M.D., Prof. Chemistry in the University of New 

 York. 



"YTERY soon after M. Daguerre's remarkable process for 

 * Photogenic Drawing was known in America, I made at- 

 tempts to accomplish its application to the execution of por- 

 traits from the life. M. Arago had already stated, in his ad- 

 dress to the Chamber of Deputies, that M. Daguerre expected, 

 by a slight advance, to meet with success, but as yet no ac- 

 count has reached us of that object being attained. 



More than one hundred instances are recordedin Berzelius's 

 chemistry, in which the agency of light brings about changes 

 in bodies ; these are of all kinds : formations of new com- 

 pounds, re-arrangements of elements already in union, changes 

 of crystallographic character, decompositions, and mechanical 

 modifications. 



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 pre- 

 sently describe, of galvanization, are to free the plate from its 

 sensitive coating, and in no wise affect the depth of the sha- 

 dows, as some of the French chemists at first supposed. 



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? 



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

 bearing. 



(a.) It has long been known, that if a piece of soapstone or 



