292 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK m. 



'Many improvements were afterwards made in this process, which 

 is now no longer practised, having been replaced by many others, 

 more expeditious and less costly ; but from the first the daguerre- 

 otype proofs attained a finish, a precision which have never since been 

 surpassed. In a historical and scientific point of view, however, 

 and as an application of the Jaws of physical phenomena, Daguerre's 

 process has an importance which necessitates our describing it 

 in detail. The enthusiasm with which it was received at the outset 

 by savants and by the public, as well as by artists, was but its due, if 

 we consider the immense services it has rendered, and which the new 

 processes render still more. Geography, the physical and natural 

 sciences, ethnology, architecture, and even the arts of drawing and 

 painting, have been indebted to and have benefited from the aid of 

 photography. 



Let us see, then, what was the original process of Daguerre in 1839, 

 and how he succeeded in reproducing pictures by means of the 

 process which was then called the daguerreotype. 



II. THE DAGUERREOTYPE. 



Daguerre employed, like Niepce, a sheet of copper of the thickness of 

 strong cardboard plated with silver. He divided into five operations the 

 series of manipulations which formed his process. The following de- 

 scription of them is taken from the notice published by the inventor. 



The first operation consisted in cleaning and polishing the plate. 

 The silver surface was first polished very carefully with some cotton 

 steeped in olive oil and some very finely powdered pounce ; the greasy 

 coating was then taken off with a stopple moistened with nitric acid 

 and water. The plate, made very hot, was again polished with pounce, 

 this time dry, until the silver became perfectly bright. 



In this state the plate was ready to receive the sensitizing bath, a 

 second operation, which consisted in exposing the polished surface to 

 the vapours spontaneously exhaled from some fragments of iodine. 1 

 This was done in darkness, and the operator could only judge by the 



1 It should be remarked that Niepce tried to bleach his bitumen with iodine, 

 and after communicating this fact to Daguerre it is probable that this savant first 

 observed the action of light on iodide of silver after repeating the experiment. 



