174 CHEMICAL AND MOLECULAR 



that a disturbance lias been produced upon the portions 

 which were illuminated, whereas no change can be 

 detected upon the parts which were kept in the dark. 

 It was thought, until lately, that a few chemical com- 

 pqunds, such as the iodide of silver, the material em- 

 ployed in the Daguerreotype and Calotype, chloride of 

 silver, the ordinary photographic agent, a few salts of 

 gold, and one or two of lead and iron, were the only 

 materials upon which these very remarkable changes 

 were produced. We now know that it is impossible to 

 expose any body, simple or compound, to the sun's rays, 

 without its being influenced by this chemical and mole- 

 cular disturbing power. To take our examples from 

 inorganic nature, the granite rock which presents its 

 uplifted head in firmness to the driving storm, the stones 

 which genius has framed into forms of arcliitectural 

 beauty, or the metal which is intended to commemorate 

 the great acts of man, and which in the human form 

 proclaims the hero's deeds and the artist's talent, are 

 all alike destructively acted upon during the hours of 

 sunshine, and, but for provisions of nature no less 

 wonderful, would soon perish under the delicate touch 

 of the most subtile of the agencies of the universe. 



Niepce was the first to show that all bodies which 

 underwent this change during daylight possessed the 

 power of restoring themselves to their original con- 

 ditions during the hours of night, when this excitement 

 was no longer influencing them. Resins, the Daguer- 

 reotype plate, the unprepared metal tablet, and numerous 

 photographic preparations, prove this in a remarkable 

 manner.* 



* For several illustrations of this remarkable phenomenon, see 

 On the Action of the Rays of the Solar Spectrum on Vegetable 

 Colours, and on some new Photographic Processes ; by Sir John F. 

 W. Herschel, Bart., K.H., F.R.S. Phil. Trans. June, 1842, vol. 

 cxxxiii. ; On certain improvements on Photographic Processes 

 described in a formr communication, and on the Parathermic Rays 



