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XIX. — The Colours of the Atmosphere considered with reference to a previous 

 Paper " On the Colour of Steam under certain circumstances." By James D. 

 Forbes, Esq. F. R. SS. L. 8,- Ed., Professor qf Nattiral Philosophy in the 

 University of Edinburgh. 



Read 4th February 1839. 



In the following Paper, it is proposed to illustrate more fuUy the hint ex- 

 planatory of certain atmospheric colours, given in a notice of the remarkable red 

 hue of condensing steam, communicated on the 21st January. Since that time, I 

 have examined with care the principal authors who have adverted to the subject 

 of the colour of the sky generally, and of the redness of sunset in particular ; and 

 since, in the course of that research, I have found much to confirm, and little to 

 modify, the view which I have already taken of the subject, I hope that the pre- 

 sent Paper may be considered as a fit appendix to my former experimental notice. 

 It will be recollected that in it I stated the singular fact, that steam does not pass 

 at once from the state of invisible pellucid vapour to that of a misty white cloud, 

 such as issues fi-om the spout of a tea-kettle ; but that an intermediate stage oc- 

 curs, in which it is coloured, even very highly, giving to transmitted light a hue 

 varjdng from tawny yellow up to intense smoke-red. I then observed, that, since 

 this phenomenon does not require steam of high tension for its production, it is 

 very probable that the tints of sunset and of artificial Ughts seen through certain 

 fogs, may be owhig to the absorptive action of watery vapour in this critical 

 condition. 



Ebeehard, a -^vriter of more than sixty years ago, states that the multitude 

 of opinions of authors on the colour of the sky alarmed him when he came to 

 analyze them ; and as, since his time, these have perhaps been doubled, some idea 

 may be formed of the labom- required to collect and classify the scattered notices 

 which are to be found in special treatises, academical collections, and periodical 

 works, respecting it. The most copious references I have found amongst German 

 authors, but these I have, in almost every case, been able to verify by a reference 

 to the original authorities. The result has been a reduction to a few of those 

 authors who have added any thing of consequence to a subject which has rather 

 been one of opinion than of science, untU lately ; and to still fewer of those who, by 

 any one original observation or experiment, have added a single mite to the data 

 for reasoning. The mass of copyists I may pass over in silence, or ^vith little 

 notice, and thus I hope to be able to reduce into moderate compass the results of 

 a considerably tedious investigation. 



It is impossible to advance any consistent theory about the colours of dawn, 



VOL. XIV. PART II. 3 c 



