Professor Draper on the Process of Daguerreotype. 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. 



XX X II. On the Process of Daguerreotype, audits application 

 to taking Portraits from the Life. By JOHN WILLIAM 

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

 York. 



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 



