194 THE CALOTYPE. SECT. XXIV. 



not possess before. Metallic salts, especially those of 

 silver, whose molecules are held together by an unstable 

 equilibrium, are of all bodies the most susceptible of its 

 influence ; the effects however vary with the substances 

 employed and with the different rays of the solar spec- 

 trum, the chemical properties of which are by no means 

 alike. As early as 1772 M. Scheele showed that the 

 pure white color of chloride of silver was rapidly dark- 

 ened by the blue rays of the solar spectrum, while the 

 red rays had no effect upon it; and in 1801 M. Hitter 

 discovered that invisible rays beyond the violet extremity 

 have the property of blackening argentine salts, that 

 this property diminishes toward the less refrangible part 

 of the spectrum, and that the red rays have an opposite 

 quality, that of restoring the blackened saltaflfLsilver to 

 its original purity, from which he inferredB3gthe most 

 refrangible extremity of the spectrum ha^pn oxygen- 

 izing power, and the other that of deoxygenating. Dr. 

 Wollaston found that gum guaiacum acquires a green 

 color in the violet and blue rays, and resumes its original 



fin the red. No attempt had been made to trace 

 ural objects by means of light reflected from them 

 Mr. Wedgewood, together with Sir Humphry Davy, 

 took up the subject: they produced profiles and tracings 

 of objects on surfaces prepared with nitrate and chloride 

 of silver, but they did not succeed in rendering their 

 pictures permanent. This difficulty was overcome in 

 1814 by M. Niepce, who produced a permanent picture 

 of surrounding objects, by placing in the focus of a 

 camera obscura, a metallic plate covered with a film of 

 asphalt dissolved in oil of lavender. 



MA Fox Talbot, without any knowledge of M. Niepce's 

 experiments, had been engaged in the same pursuit, 

 and -must be regarded as an independent inventor of 

 photography, one of the most beautiful arts of modern 

 times : he was the first who succeeded in using paper 

 chemically prepared for receiving impressions from nat- 

 ural objects ; and he also discovered a method of fixing 

 permanently the impressions that is, of rendering the 

 paper insensible to any further action of light. In the 

 calotypo, one of Mr. Talbot's most recent applications 

 of the art, this photographic surface is prepared by wash- 



