THE DAGUERREOTYPE. 327 



833. EXPLANATION. The addition of a 



Explain the 



action of sul- few drops of sulphuric acid, produces the 

 above-mentioned change instantaneously. 

 This acid combines with the potassa, 

 setting the manganic acid at liberty. One portion of 

 manganic acid then appropriates part of the oxygen of 

 the other part, and converts itself into hypermanganic 

 acid, which still remains combined with potassa, im- 

 parting the red color to the solution. The deoxydized 

 portion of the acid precipitates, at the same time, as bin- 

 per oxide. The remaining manganates are not of especial 

 interest or importance. 



THE DAGUERREOTYPE. 

 834. THE DAGUERREOTYPE. The da- 



Explain the 



guerreotype may be regarded as a painting 



i n mercury, upon a silver surface. The 

 employment of mercury is preceded by what may be 

 called an invisible painting upon the silver. This is ac- 

 complished, like the production of an image in a mirror, 

 by mere presentation of the picture, or other object 

 to be copied, before the prepared plate. The mercury, 

 afterward used in the form of vapor, adheres to the 

 plate, and forms its white amalgam, just in proportion 

 to the lights and shades of the previous image thrown 

 upon the plate. 



Describe the 835. THE DAGUERREOTYPE PROCESS. - 



process of ta- j n or( j er to p re p are the plate for what has 



King daguer- 



reotypes. above been called the invisible painting, it 



is exposed to vapors of iodine, and thereby covered 



