SECT. XXIV. SOLAR SPECTRUM. 203 



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Library 



SECTION XXIV. 



" Of 



Chemical or Photographic Rays of Solar Spectrum 



Wollaston's Discoveries Wedgwood's and Sir Humphry Davy's 

 graphic Pictures The Calotype The Daguerreotype The Chroma- 

 type The Cyanotype Collodion Sir John HerscheFs Discoveries in 

 the Chemical Spectrum M. BecquereFs Discoveries of Inactive Lines in 

 ditto Thermic Spectrum Phosphoric Spectrum Electrical Pro- 

 perties Parathermic Rays Moser and Hunt's Experiments Gene- 

 ral Structure and antagonist Properties of Solar Spectrum Defracted 

 Spectrum. 



THE Solar Spectrum exercises an energetic action on matter, 

 producing the most wonderful and mysterious changes on the 

 organised and unorganised creation. 



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

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

 ing properties to them which they did not possess before. Collo- 

 dion and 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. Bitter discovered that invisible 

 rays beyond the violet extremity have the property of blacken- 

 ing argentine salts, that this property 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 extremity of the spectrum has an oxygenising power, 

 and the other that of deoxygenating. Dr. Wollaston found that 

 gum guaiacum acquires a green colour in the violet and blue 

 rays, and resumes its original tint in the red. No attempt had 

 been made to trace natural objects by means of light reflected 

 from them, till Mr. Wedgwood, together with Sir Humphry 



