224 SOLAR SPECTRUM. [SECT. xxiv. 



SECTION XXIV. 



Chemical or Photographic Rays of the Solar Spectrum Messrs. Scheele's, 

 Ritter's, and Wollaston's Discoveries Mr. Wedgewood's and Sir Humphry 

 Davy's Photograph! cPictures TheCalotype, the Daguerreotype, theChro- 

 matype, the Cyanotype Sir John HerschePs Discoveries in the Photo- 

 graphic or Chemical Spectrum Mons. E . Becquerel's Discovery of Inactive 

 Lines in the Chemical Spectrum. 



THE Solar Spectrum has assumed a totally new character 

 from recent analysis, especially the chemical portion which 

 exercises an energetic action on matter, producing the most 

 wonderful and mysterious changes on the organized and un- 

 organized creation. 



All bodies are probably affected by light, but it acts with 

 greatest energy on such as are of weak chemical affinity, im- 

 parting properties to them which they did 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 spectrum, the chemical properties of which 

 are by no means alike. As early as 1772 M. Scheele showed 

 that the pure white colour of chloride of silver was rapidly 

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

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

 covered that invisible rays beyond the violet extremity have 

 the property of blackening argentine salts, that this pro- 

 perty diminishes towards the less refrangible part of the 

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

 that of restoring the blackened salt of silver to its original 

 purity, from which he inferred that the most refrangible ex- 

 tremity of the spectrum has an oxygenizing power, and the 

 other that of deoxygenating. Dr. Wollaston found that 



