GASEOUS SPECTRA UNDER HIGH DISPERSION. 431 



illumination, once begins in a CH tube, it never stops until all the H has 

 freed itself from the trammels of connection with C (Carbon) ; and that 

 element either tails inert, or if it can find any O, combines with that, and 

 appears as CO, to the still further confusion of all CH bands. 



At the same making where these two defiant gas tubes at 2""0 and 4" - 

 pressure were prepared, Mr Casella made for me another tube at only O'l 

 pressure. There was no O nor CO visible there, nor any CH either ; nothing 

 but the most brilliant set of lines of pure and simple H that were ever beheld, 

 I should suppose, by mortal eye. In fact, at that low pressure, the first spark 

 had decomposed the whole of that faint charge of Olefiant gas ; its C was 

 nowhere visible, but its H atoms were vibrating everywhere : and the only 

 consolation I had for seeing nothing of the expected CH was, the apposite 

 illustration that the whole case offered of a favourite idea of the late excellent 

 Sir William Siemens, whose loss we all deplore. His idea being, that the 

 gases which, by combining under 800 inches pressure on the surface of the 

 Sun, give out light and heat, — may, when excessively rarefied by removal into 

 outer space, become decomposed or separated from each other under even the 

 weakest physical influences ; but are made ready in that way, on their return 

 to the Sun, to give out light and heat by renewed combination under pressure, 



over again. 



Blue and Violet CH Bands. 



Of the Blue band of CH, I have little to say beyond what my readers will 

 find out for themselves, on referring to Plate LIV., to my Index Map (Plate 

 LXXVIL), and also to MM. Angstrom and Thalen's Index Map, which mutatis 

 mutandis is fairly enough compatible therewith. 



But in the case of the Violet band, so large and bulky with me, so 

 thin, small, and vanishing with them, there is a huge difference to be ex- 

 plained. 



Now I should have already indicated that an exactly opposite difference in 

 the case of the Orange band, seemed to be attributable there to the Swedish 

 observations being made on gases at far greater pressure (probably the full 

 atmospheric) than the densest fillings of any of my tubes ; and the same reason, 

 though with opposite effects, is apparently the acting cause at the opposite, or 

 violet, end of the spectrum, of what we may note there. For with my own 

 tubes, the denser the filling, the nearer did the Orange band come to the 

 larger Swedish development of it, — yet the nearer also did the Violet band 

 come to the Swedish depreciation of it. In fact in my densest tubes I have 

 not only found the Violet band (the furthest visible one of the Coal-gas 

 Blow-pipe series) an almost vanishing quantity, — but have proved an entire 

 absence of a still more beautiful and powerful band beyond it — and which 



VOL. XXXII. PART III. 4 A 



