GASEOUS SPECTRA UNDER HIGH DISPERSION. 429 



the Violet band in the Upsala Memoir, — so very differently to what occurs in 

 that grand constant, viz., the Blow-pipe flame's spectrum of coal-gas and air. 



In this last material nothing is easier on any occasion, and for any 

 length of time, than to get all 5 bands to show ; but the Orange band is 

 always weak; and did therefore altogether escape some of the earlier 

 observers. 



In the tubes also, by electric light, the Orange band is far weaker than 

 either the Citron, or the Green ; but it has another difficulty to contend against 

 there, of this nature, — 



When the pressure of the CH gas is small, it is so very easily decomposed 

 by the electric spark, that Hydrogen low-temperature lines are set free ; and 

 being nowhere stronger and more multitudinous than over the Orange region, 

 they completely mask any residual traces that may remain of Orange CH, and 

 much of the Citron CH, band, as well. But, as I have been finding with tubes 

 specially prepared to that end, the decomposition becomes less and less with 

 increasing density of the filling, until at several whole inches, instead of 

 hundredths of an inch as with the old tubes, — Hydrogen lines nearly disappear, 

 and CH bands like the blow-pipe's bands, so far as their range extends, are 

 almost the only existences visible. 



To get the CH Orange band, however, quite clear of those obstructions, is 

 particularly difficult. Thus with coal-gas at 5 inches pressure, last year, every 

 blow-pipe band was well seen, except the unfortunate Orange one ; and so it 

 was also this year with a fine tube of Olefiant gas prepared by Mr Casella at 

 2 inches pressure. But with another tube he had prepared at the same making, 

 at 4 inches pressure, — such is the superiority of Olefiant, to Coal, gas for 

 this purpose, — the long desired cynosure was reached at last. For in that 

 tube's spectrum, while not a single low-temperature H line appeared, there was 

 the Orange CH band as perfect in its symmetry of lines and linelets as any- 

 thing could well be imagined, and as I have never seen it written yet. 



There were to be counted in it 5 leading lines very bright and distinct and 

 perhaps a sixth, all at successively smaller intervals in proceeding towards the 

 violet : while between every pair of them, and in the interval beyond of greater 

 refrangibility, were the linelets, in infinite thinness, sharpness, and definition ; 

 at first, or towards the red side, very close set, but continually increasing their 

 distances apart, and preserving their inimitable Liliputian visibility, right up to 

 the very beginning of the Citron band. This being a glorious extension of the 

 vanishing side of the Orange band, far beyond anything ever seen with the 

 blow-pipe, or even with tubes, when coal-gas is the filling medium. 



Next, turning to the Citron band of CH (in the same 4 inch pressure 

 Olefiant gas tube) its leading lines were vividly bright ; its linelets also un- 

 precedentedly clear, and never ceasing for a moment until, although continually 



