294 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK m. 



which would go on decomposing and destroy the proof if it remained 

 too long exposed to the light." Was this interpretation scientifically 

 exact ? We shall see later on. The inventor always succeeded 

 in his aim by shaking the plate in a hot solution of sea-salt, or better 

 still, in a solution of hyposulphite of soda, 1 and then washing it in 

 very hot water. When all trace of the golden coating had disap- 

 peared, this last operation was known to be successful. The proof 

 was then covered with glass to save the surface from being scratched 

 or rubbed, and it was thus preserved intact, even when exposed to the 

 light. 



This is an epitome of the method invented by Daguerre without the 

 details of manipulation, which are devoid of interest from a scientific 

 point of view. Improvements and processes were soon added which 

 ere long dethroned the original invention, without in any way detract- 

 ing from the merit of the two men who contributed to its discovery. 



IT. IMPROVEMENTS MADE IN DAGUERRE'S PROCESS. 



We have shown with what enthusiasm the discovery of Niepce 

 and Daguerre was everywhere received. As the manipulations re- 

 quired in this art were neither difficult nor expensive, and as, thanks 

 to the law, the invention had become public property, a number of 

 amateurs, artists, and scientific men set themselves to practise 

 photography. The result was a series of modifications and improve- 

 ments on the original method. We shall only mention the most 

 important of these advances. From the outset attention had been 

 directed to making the images as lasting as possible by protecting 

 them from friction and from the ulterior action of light. M. Dumas 

 proposed covering the plate with varnish, by pouring on the surface a 

 boiling solution of one part of British gurn in five parts of water. 

 Mention must also be made of M. Fizeau's fixing with chloride of gold. 

 After having carefully washed the plate in hyposulphite of soda, M. 

 Fizeau poured over the whole surface a mixed solution of chloride of 

 gold and hyposulphite of soda, then he heated the plate underneath with 



1 The solvent action of hyposulphite of soda had been discovered by Sir John 

 Herschel in 1819. The introduction of this salt for fixing was long subsequent to 

 the discovery of the daguerreotype. 



