92 Professor Deivar [April 1, 



are not simply due to differences of temperature. In fact we know so 

 little about the mechanism, so to speak, of the changes of electric 

 energy into heat, and of heat into radiation, that there is no good 

 reason for assuming that the energy which takes the form of radiation 

 in the electric discharge through a gas must first take the form of 

 the motion of translation of the particles on which temperature 

 depends. The gas may, for a short time, be intensely luminous at a 

 very low temperature ; and if the impulses which give rise to the 

 vibratory movements of the particles be of different characters, the 

 characters of the vibrations also may differ within certain limits. 



Leaving, however, the realms of speculation, we may mention 

 that we have before observed the spectrum of iron at a temperature 

 intermediate between that of the oxyhydrogen-jet and that of the 

 electric arc. 



Some time since (' Proc. E. S.' xxxiv. p. 119, and ' Proc. Camb. 

 Phil. Soc' iv. p. 256) we described the spectrum proceeding from the 

 interior of a carbon tube strongly heated by the electric arc playing 

 on the outside. This spectrum approaches more nearly to that of the 

 arc inasmuch as it shows all, or almost all, the iron lines given by 

 the arc betw^een F and 0, and the aluminium pair between H and K ; 

 but it resembles the explosion-spectrum in the relative strength of 

 some of the iron lines, and in the absence of almost all iron lines 

 between and T. The iron lines seen reversed against the hot walls 

 of the carbon-tube correspond with the strongest of the explosion- 

 lines ; the strong lines near M and a little below L in the explosions 

 being those most strongly reversed in the photographs of the carbon- 

 tube. The greater comjilctencss and extent of the iron spectrum, as 

 well as the presence of the aluminium lines, which are entirely 

 wanting in the explosion-spectrum, indicate that the temperature of 

 the tube was higher than that of the explosion. That iron, nickel, 

 and cobalt are volatile in some degree at the temperature of the 

 explosion appears to be proved, and makes the api)earance of iron 

 lines at the very apices of solar prominences, as observed by Young, 

 less astounding than it seemed to be at first sight. The ascending 

 current of gas making the prominence may very well carry iron 

 vapour with it ; or we may not unreasonably suppose that there is 

 meteoric dust containing iron everywhere in the outer atmosphere of 

 the sun, which becomes volatilised, and emits the radiation observed, 

 when it is heated up by the hot current of the prominence. What 

 the temperature of such a current may be we cannot well gauge, but 

 it is high enough to give the hydrogen-spectrum, of which no trace 

 has been observed in the flash of the exi^losions or in the oxy-hydrogen- 

 jet. The temperature of the explosions we know with tolerable 

 accuracy, at least when the gases are at atmospheric pressure to 

 begin with. Bunsen (' Phil. Mag.' 1867, p. 494) found the pressure 

 of the explosion W' as for hydrogen and oxygen 9 • 6 atmosj)heres, 

 and for carbonic oxide and oxygen 10*3 atmospheres, and he 

 calculated the corresponding temperatures to be 2844° and 3033°. 



