34< Professor Forbes on the Colours of the Atmosphe^-e. 



pure air its greatest transparency, — next, the transition state, 

 when, still invisible in form, and almost certainly not vesicular, 

 it transmits a steady orange glare, not the play of colour which 

 is often seen in clouds and fogs forming a glory round a ra- 

 diant body ; — and lastly, the vesicular steam, such as we every 

 day see issuing from the spout of a tea-kettle reflecting irides- 

 cent colours, just as the semi-opake clouds do which seem to 

 float across the disk of the sun or moon. These coronae, not- 

 withstanding their apparent analogy to the colours of thin 

 plates, seem rather to be due to the effect of diffraction*. 



The non-appearance of the lines of the spectrum in my 

 experiment, may be plausibly explained in the following man- 

 ner, which, however, I offer merely as a conjecture. When 

 steam of high pressure issues from an orifice, a horizontal 

 section of the expelled column will include vapour in every 

 stage of condensation. Its centre, up to a certain height, 

 will be pure invisible steam ; at the exterior of all, in contact 

 with the cold air, there will manifestly be vesicular steam, and 

 a cylindrical space between the two will contain red steam. 

 Now it is extremely probable, that when the experiment is 

 performed on the small scale, as I have described it, by suf- 

 ferinfT light to pass through such a compound column, and 

 then analysing it by the prism, enough of unabsorbed rays 

 are reflected from the highly luminous surface of the vesicular 

 steam to prevent the fine lines from being seen if they exist. 

 And I am strongly confirmed in this conjecture by the fact, 

 that when the rush of steam is very violent, and always when 

 much vesicular vapour is present, the unabsorbed part of the 

 spectrum presents a washy and impure tint (particulai'ly men- 

 tioned in my former paper), which probably arises from a 

 blending of the colours, produced by this cause. 



In conclusion, I have only a word or two to say respecting 

 the application of these facts to atmospheric appearances re- 

 garded as prognostics of weather. The modified hues of the 

 sky, and of the sun and moon near the horizon, have, for so 

 many ages, and in so many countries, been regarded as the 

 surest indications of atmospheric changes, that we cannot 

 doubt that it is to the variety of conditions in which vapour 

 exists in the air," more or less nearly condensed, that these 

 pha;nomena are due. Humboldt describes the colour and 

 form of the sun's disc at setting in tropical regions, as the 

 most infallible prognosticfj and elsewhere ascribes these va- 

 riations "to a particular state of the vesicular vapour J." 



* See Young's article ChromaticSy in Encyc. Brit., and Fraunhofer in 

 Schumacher's Astronomische Abhandlungen. Drittes Heft, 1825. 

 t Relation Historique, 8vo, ii. 328, \ New Spain (translation), ii, 326. 



