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Royal Society 



colour from a deep brownish red, through different shades of brown, 

 to a more or less pure yellow, according to the thickness and nature 

 of the film ; for, as is shown in the paper " On the Polarization of 

 Light by Crystals of Iodine " (Proc. Koy. Soc. vol. xxv. p. 51), the 

 colour of the transmitted light apparently does not depend solely 

 on the thickness of the layer of iodine through which it passes. 

 These films correspond in colour with alcoholic solutions of iodine of 

 different strength; and the absorption- spectra are very similar — the 

 whole of the blue end of the spectrum being cut off, and the absorp- 

 tion extending further and further towards the less-refrangible end 

 of the spectrum, as the thickness of the film increases, till at length 

 only light having a wave-length of about 650 (in " tenth-metres "), 

 or slightly more refrangible than the C line, passes through ; and 

 a very slight increase in the thickness of the film is sufficient to 

 stop this also. 



Pig. 4 shows the appearance with a film of medium thickness, 

 the strong absorption ending at a point about one third of the 

 distance between E and D, together with a certain amount of dark- 

 ening extending to about D, and represents the mean results from 

 measurements of the absorption-spectra of seven different films of 

 iodine. 



Liquid Iodine. 



With a little careful management it is usually possible to melt 

 one of these thin layers of iodine by heating it over the flame of 

 a spirit-lamp, without causing it either to be dissipated in vapour 

 or to run together. The liquid iodine appears to be more trans- 

 parent than the solid ; for a layer which appears a deep red 

 colour by transmitted light when hot, sometimes becomes perfectly 

 opaque on cooling. When hot it is also of a deeper red than when 

 cold ; and the spectroscope shows that while the less-refrangible 

 rays of the spectrum are freely transmitted, there is more absorp- 

 tion of those of mean refrangibility than is the case with solid 

 iodine. * 



Pig. 3 shows the absorption- spectrum as deduced from five obser- 

 vations. Just as is the case with solid iodine, light of a wave- 

 length of about 650 suffers the least amount of absorption during 

 its passage through a layer of iodine. 



