In the early part of the 19th century Daguerre created the first photographic plate. It consisted of a thin film of polished silver on a copper base, the silver surface was sensitized by putting it into a container with iodine, the iodine vapors reacted with the polished silver surface and formed a thin yellow layer of silver iodide. After a photograph was taken on the plate, it was developed by exposing the plate to magnesium vapor at 150 degrees F. The vapor would only stick to the parts of the plate which had been exposed to the light. The plate was then dipped in sodium thiosulfate to dissolve the unused silver iodide, and then rinsed in hot water. Daguerreotypes, as these images were called, had the ability to capture fine detail but due to their long exposure time they were constrained to motionless, or very patient, subjects.
On 4 January 1829 Niépce agreed to go into partnership with Louis Daguerre. Niépce died only four years later, but Daguerre continued to experiment. Soon he had discovered a way of developing photographic plates, a process which greatly reduced the exposure time from 8 hours down to half an hour. He also discovered that an image could be made permanent, or "fixed", by puting it in salt.
Following a report on this invention by Paul Delaroche, a leading scholar of the day, the French government bought the rights to it in July 1839. Details of the process were made public in August of 1839, and Daguerre named it the Daguerreotype.
The announcement that the Daguerreotype "requires no knowledge of drawing...." and that "anyone may succeed ... and perform as well as the author of the invention" was greeted with interest, and "Daguerreomania" became a craze. The Daguerreotype, though good quality, was expensive. The picture could not be reproduced either, but this was not regarded as a disadvantage because the owner then had a one of a kind work of art.
The Daguerreian Society - http://www.daguerre.org/resource/texts.html
Mirror Image Gallery - http://lb-web.com/daguerreotypes/
Post-Impressionism - http://www.msjc.edu/art/djohnson/art100/100lecture12.html