:244 J. Trowbridge — Gaseous Constitution of the 



less than two inches from either terminal. The photograph 

 therefore of the spectrum produced by a spark of three or four 

 feet in length, taken at the middle of the spark, shows nothing 

 but air lines, whatever may be the nature of the terminals. 

 Indeed this is true even when sparks of eight inches in length 

 are examined. The most effective way, therefore, of sifting 

 out air lines from metallic spectra is to employ long and very 

 powerful sparks. At one time I believed that the oscillatory 

 nature of the discharge influenced the character of the spectra ; 

 I am now of the opinion that introducing self-induction acts 

 by diminishing the energy of the discharge ; and that the rate 

 of oscillation has very little if any effect. 



In the present paper I shall confine my attention largely to 

 the immediate region of the H.H. lines of the solar spectrum. 



In Geissler tubes, also, having capillaries not less than two 

 inches in length, the terminals being three and a half or four 

 inches apart, no metallic lines were observed under the condi- 

 tions of my work. Indeed, to produce metallic spectra at such 

 distances from the electrodes during the time of duration of 

 the discharges, would demand a prodigious velocity of the 

 metallic particles. I have made a careful study of the influence 

 of metallic electrodes in the tubes employed by me and find no 

 spectra due to them. 



In order to determine whether the walls of the glass tubes 

 could give lines due to calcium, I first placed aluminum ter- 

 minals on a sheet of glass of the same kind as that from which the 

 Geissler tubes were made, and having placed the glass against 

 the slit of the spectroscope I passed powerful discharges of the 

 same nature and energy as were employed in the study of the 

 spectra of gases. The glass was badly corroded along the path 

 of the discharge, showing the same corrosion which was 

 observed in the capillary of the glass Geissler tubes. 



!N"o continuous spectrum was observed and no calcium lines. 

 Similar discharges were passed through fifty ohms of No. 36 

 iron wire ; the wire was barely raised to a dull red heat. 

 A photograph was taken of ten centimeters of such wire, 

 illuminated by the discharge from a Geissler tube placed in 

 the same electrical circuit. The photograph showed the wire 

 intact at the moment of the illumination of the tube. It took 

 time to communicate sufficient heat to melt the wire. On the 

 same photograph was shown the subsequent melting of the 

 wire; that is, the wire is seen intact and also the two ends of 

 the wire contorted and burning. If the walls of the capillary 

 of the glass vessels are heated to incandescence, the time ele- 

 ment must be large; for the gas must first be heated by the 

 discharge and then the walls of the glass by conduction and 

 radiation. Thermodynamic considerations make it impossible 



