Absorption and Radiation of Heat by Gases and Vapours. 273 



of coloured rings by the interference of these pencils, is certainly 

 one of the most remarkable facts in physical optics ; and, in a 

 theoretical point of view, no less remarkable is the fact that one 

 of the interfering portions is a fasciculus of pencils returned into 

 the refracted beam by different routes, and having different 

 origins. 



Owing to the extreme thinness of the combined films, we 

 cannot, as with thick plates of uniaxal crystals, see at once the 

 black cross and its attendant rings ; but in numerous specimens 

 of decomposed glas3 to which I have already referred, the films 

 are spherical shells of different diameters and thicknesses, and 

 exhibit the black cross with the greatest sharpness and beauty. 

 In many specimens these circular combinations are perfectly 

 colourless, and the colours of the four luminous sectors which 

 embrace the black cross rise only to the white of the first order. 



When the films are so thin as to give the colours of thin 

 plates, the colour of the luminous sectors is generally the same 

 as that of the film in which the circular portions occur, and the 

 rings or bands which surround them have a very peculiar cha- 

 racter, owing to the manner in which the spherical shells are 

 joined to the films which compose the plate. 



How far these results may lead to new views of the struc- 

 ture which produces double refraction, it would be unprofitable 

 to inquire in the present state of our knowledge of the atomical 

 constitution of transparent bodies. 



XXXVI. On the Absorption and Radiation of Heat by Gases and 

 Vapours, and on the Physical Connexion of Radiation, Absorp- 

 tion, and Conduction. — The Bakerian Lecture. By John 

 Tyndall, Esq., F.R.S. Z$c. 



[Concluded from p. 194. J 



§ ^' ACTION of permanent Gases on Radiant Heat.— The 

 •^*- deportment of oxygen, nitrogen, hydrogen, atmospheric 

 air, and olefiant gas has been already recorded. Besides these 

 I have examined carbonic oxide, carbonic acid, sulphuretted 

 hydrogen, and nitrous oxide. The action of these gases is so 

 much feebler than that of any of the vapours referred to in the 

 last section, that, in examining the relationship between absorp- 

 tion and density, the measures used with the vapours were aban- 

 doned, and the quantities of gas admitted were measured by the 

 depression of the mercurial gauge. 



