432 C. PIAZZI SMYTH ON MICROMETRICAL MEASURES OF 



ought otherwise (as well as another band between the blue and the violet) to 

 be always seen at electric temperature, viz., 



The Marsh- Violet CH Band. 



This band was so-called, from Professor Alexander Herschel first finding 

 it during some of these experiments in one of my tubes of Marsh gas ; but it 

 was already known to older spectroscopists who have used pure Oxygen, in 

 place of atmospheric air, in their blow-pipes. 



With stronger sparks too than my earlier ones, I have latterly found the 

 band developed to more or less extent in every kind of CH gas ; — and capable 

 of coming out Avith far more force and picturesque luminosity that the previous 

 Violet, or the ante-previous Indigo, band. 



In short with its very pronounced leading lines, and then the expanding 

 linelets after each of them, in such regular series, — this last and latest " Marsh 

 Violet " CH band may be considered a most typical example of a CH band. 

 It would also probably make a still more magnificent appearance in 

 photography ; — for, it is so far within the ultra violet of the spectrum range, as 

 not to exhibit its full glories to the human eye, but is by just so much within 

 the sphere of the impressibility of bromo-ioclide of silver, focussed on by quartz 

 lenses and prisms. My own plate of it therefore (No. LIV.) must be looked on 

 as its interim presentation only. 



Of the Chemical Interpretation of this CH Spectrum. 



Through all the variations I have been describing of this CH spectrum, 

 however much more or less may have been visible at its one end, or the other, 

 by reason of accompanying circumstances just explained, — no one known and 

 recognised band in it, when tested by its sharp leading lines has been moved 

 out of its spectrum place by the smallest, recognisable quantity. Hence it is 

 one and the same spectrum throughout all the above intensity variations, and 

 one so continually met with in this world, and in astronomy is so characteristic 

 of the self-luminosity of comets, that it is most important to know wheat 

 chemical science says as to its origination and nature. 



I have been, thus far in the present paper, calling it the CH, or Carbo- 

 Hydrogen, spectrum ; but in London among the magnates of Chemistry and 

 Spectroscopy, it has been declared to be the spectrum of C, or pure 

 Carbon alone. 



So, too, it was evidently very firmly held to be by them, a few years ago. 

 For when I sent a paper on Auroral Spectroscopy to the Royal Astronomical 

 Society in 1871 making use of the Candle-spectrum as a reference, and attri- 

 buting it to CH in general, and Acetylene, or C 2 H 2 , in particular, — I have been 



