1889.] of the various Species of Heavenly Bodies. 213 



indubitable proof of the inherent luminosity of this body, and also of 

 a greater complication of chemical constitution than former observa- 

 tions had implied. It seems to be a particularly noteworthy fact 

 that the usual cometary spectrum observed first by Tacchini and 

 Vogel from May 22nd to 31st disappeared, while in its stead the 

 bright line spectrum was developed. As this occurrence coincides 

 with the approach of the comet to perihelion, the cause of it may be 

 sought in the rapidly increasing heat of the comet, as thereby on the 

 one hand the sodium present in it was turned into vapour, and on the 

 other hand the electric processes within its mass attained greater 

 vigour. From a discussion of the earlier spectroscopic observations 

 of the comet, and from comparative laboratory experiments of the 

 spectral relations of hydrocarbon, it seems to me very probable that 

 the development of light within this comet chiefly depended on disrup- 

 tive electric discharges."* 



Hasselberg further refers to the experiments of E. Wiedemann on 

 the spectra observed during the passage of an electric current through 

 mixed gases and vapours. 



Wiedemann found that when electric sparks were passed through a 

 heated tube containing sodium and a gas like hydrogen or nitrogen, 

 the spectrum consisted solely of lines of sodium. Hasselberg also 

 repeated this experiment, substituting hydrocarbon for hydrogen or 

 nitrogen, and found that the same thing happened. He concludes, 

 therefore, that this demonstrates the electrical origin of the light of 

 comets, since the additional heat due to the approach of the comet to 

 perihelion might certainly bring out the sodium, but could not have 

 caused the hydrocarbon spectrum to disappear. 



I would suggest, however, that the changes which took place in 

 Comet Wells can be equally well explained on the supposition that 

 heat alone was in question. The main point to be explained is the 

 disappearance of the carbon fluting spectrum and the appearance of 

 sodium as the comet approached perihelion. With the first increase 

 in temperature, as the comet left aphelion, the occluded compounds 

 of carbon would be driven out of the meteorites constituting the 

 head of the comet, and the spectrum would consequently be that 

 of carbon. At the increased temperature due to further approach 

 to the sun, the carbon flutings would be masked by the increased 

 brightness of the continuous spectrum and by the radiation of other 

 vapours. At the same time a still larger number of meteorites 

 would become incandescent, and vapours of sodium, and possibly also 

 of iron, would distil out. Also since the stones would remain in this 

 condition for a considerable time, sodium vapour would continue to be 

 visible until they had almost ceased to be incandescent. 



I may here state that sodium exists only in very small quantities 

 * ' Astr. Nachr.,' No. 2411. 



