ABSORPTION OF LIGHT. 



Chemistry affords numerous examples of a very slight 

 change of condition, producing absolute opacity in fluids 

 which were previously diaphanous.* 



Charcoal absorbs all the light which falls upon it, but 

 in some of its states of combination, and in the diamond, 

 which is pure carb on, it is highly transparent. Gold 

 and silver beaten into thin leaves are permeated by the 

 green and blue rays, and the metals in combination with 

 acids are all of them more or less transparent. What 

 becomes of the light which falls upon and is absorbed 

 by bodies, is a question which we cannot yet, notwith- 

 standing the extensive observations that have been made 

 by some of the most gifted of men, answer satisfactorily. 

 In all probability, as already stated, it is permanently 

 retained within their substances ; and many of the expe- 

 riments of exciting light in bodies Avhen in perfect dark- 

 ness, by the electric spark and other means, appear to 

 support the idea of light becoming latent or hidden. 



No body is absolutely transparent ; some light is lost 

 in passing even through ethereal space, and still more in 

 traversing our atmosphere. 



Amongst the most curious instances of absorption is 

 that which is uniformly discovered in the solar spectrum, 

 particularly when we examine it with a telescope. We 

 then find that the coloured rays are crossed by a great 

 number of dark bands or lines, giving no light ; these 

 are generally called Fraunhofer's dark lines, as it was to 

 the indefatigable exertions of that experimentalist, and 

 by the aid of his beautiful instruments, that most of them 

 were discovered and measured, and enumerated, although 

 they were previously noticed by Dr. Wollaston. It is 

 quite clear that those lines represent rays which have 



* For examples of this, consult Graham's Elements of Che- 

 mistry ; Brande's Manual of Chemistry; or, indeed, any work 

 treating of the science. The formation of ink, by mixing two 

 colourless solutions, one of gallic acid and another of sulphate of 

 iron, may be taken as a familiar instance. 



