The so-called high pressure “ CO ” bands—or high pressure carbon bands, as they are better called—were first found by Fowler* in 1910 in tubes containing carbon monoxide at relatively high pressures. The system was described as consisting of some six apparently double-headed bands degraded to the violet, their wave-lengths being approximately at— 6441 6420 } 5897 5878 } 5431 5413 } 5030 5015 } 4679 4663 } 4365 4353 } Å. U. In 1923 the conditions of production of this spectrum were further investigated by Merton and Johnson who obtained the bands with considerable strength by condensed discharges in capillary tubes fitted with carbon electrodes, and containing CO at pressures of 5 mm. and more. It was found that while the high pressure bands and the Swan bands were mingled in the light from the capillary of the tube, the former bands were isolated in bluish jets where the two ends of the capillary merged into the wider parts of the tube. Further observations indicated that the introduction of a little C0
2
destroyed the bands, but that the system re-apppeared after a few minutes, in which time presumably the carbon dioxide had been reduced to monoxide by the carbon electrodes. A reproduction of these bands photographed under low dispersion is given in the above-mentioned paper. No further experimental work appears to have been done on this system, and it has not been correlated with any other band system or assigned any place in the system of electronic levels of the CO molecule. We have therefore made an attempt to photograph the system under high dispersion with a view to fine structure analysis and identification of the molecular emitter. For this purpose large discharge tubes having a bore of about 15 to 20 mm. and a length of 60 or 70 cm. were used. These had at least one of the electrodes made of carbon and were fitted with side bulbs containing caustic potash and phosphorus pentoxide and a palladium regulator. The tubes were filled with carbon monoxide to such a pressure (probably 20-40 mm.) that a condensed discharge could just be forced through by the ¼ kilowatt 15,000 volt transformer used. Some of the tubes had large side flasks attached to them, increasing thereby the volume of gas in the tube, and giving the tubes a life of 4 to 6 hours during which the high pressure bands were emitted strongly. After some such period the pressure fell below the optimum value, and deposits of carbon had accumulated on the walls of the tube. Impurities such as hydrogen, carbon dioxide, and water-vapour were found to inhibit formation of the high pressure bands, and the tube always attained its best condition after running for about an hour (removing meanwhile any little hydrogen present through the regulator). Under these conditions the wide bore is practically filled with light, and presents a remarkable appearance, as of dense pale blue puffs of smoke (showing the high pressure system), threaded by a narrow green ribbon (showing the Swan system). If side tubes having a fair capacity (
e
.
g
., flasks) are attached to the discharge tube the high pressure glow is capable of diffusion into these. The appearance is suggestive of an afterglow emitter, but if this is its true nature it is of very short duration. Photographs of the H. P. bands were taken in times varying from 4 to 10 hours in the first order of a 21-foot grating. The green band in the neighbourhood of λ 5000 is exceedingly faint and was not attempted. Before considering the results -obtained it will be an advantage to summarise our present knowledge of the Swan spectrum and its emitter, with which it will subsequently be shown that the high pressure carbon system is intimately related.
The structure of the high pressure carbon bands and the swan system
