GASEOUS SPECTRA UNDER HIGH DISPERSION. 439 



Remaining Bands of CO. 



Of the Red, the Scarlet, the Orange, the Yellow, and the Citron bands of 

 CO below that Green band which we have just been discussing ; and of the 

 Blue, the Indigo, and the Violet bands above the Green, — and which are all 

 pictured in the Plates ]N"os. LVIII. and LIX., — their linelets seem to have 

 somewhat similar characteristics on the whole to those of the Green band, — 

 but with compound variations, — not yet fully made out by observation. Nor 

 perhaps very soon likely to be much further elucidated, because 



(1) The dispersion of my present Prisms below the Green is too small; 



(2) Above the Green the definition is not sufficiently good ; 



(3) Towards either end of the Spectrum the illumination of my existing 

 sparking apparatus is not sufficiently bright; and 



(4) It is very difficult to get those bands perfectly free from impurities of 

 CH, H, and other gases. 



I will therefore at present proceed to a provisional termination of the CO 

 subject, by means of a few words on some general characteristics of that 

 compound gas in vacuum tubes. 



A small pressure of the gas, say 025 inch, seems to be most suitable for 

 securing a maximum of brilliancy conjoined to stability. For higher pressures, 

 say 1 inch, 2*5, 5 - 0, or 12*5 inches, simply show the same spectrum, but fainter 

 and fainter as the pressure is greater ; while lower pressures, say O'l inch or 

 under that, though exceedingly brilliant for a time, are very apt to get their 

 tubes overheated and loosened at their electrodes with loss of illuminating 

 power altogether. To prevent this catastrophe, the electrode ends of the 

 tubes, whether with platinum wires as usual passing through them, or coated 

 only with a film of silver outside, have been made to dip into vulcanite 

 insulated basins of water, and receive their electric charge from thence ; but 

 the illumination was never at its best in that manner, the whole apparatus 

 sometimes became inconveniently charged ; and with the silver coated tubes, 

 the glass was actually perforated sooner or later. Some of the best exhibi- 

 tions, however, of the CO spectrum, have been the unintended ones; as of tubes 

 prepared with Oxygen alone ; and showing at first the Oxygen spectrum, 

 but that changing during use into CO; and always more and more quickly or 

 inevitably, the weaker the pressure at which the Oxygen had been sealed in. 



Whence comes the C for this transformation of O into CO ? 



Some persons have suggested, from the use of a coal-gas, in place of a 

 hydrogen, blow-pipe in working the glass ; and an extraordinary hypothesis has 

 been recently started in Germany, of Si being convertible into C in vacuum tubes.* 



* The following note has been furnished to me: — "Herr Wessendonk prepared Siliceous gas with 

 most scrupulous care and purity, without being able to obtain a trace of Silica lines, only CO bands over 

 and over again, and more and more brilliant the purer the gas he used. Silica could never be found, 



VOL. XXXII. PART III. 4 B 



