of the Metals developed hy Exploding Gases. Ill 



Of the green and bine lines of iron seen by us in explosions 

 nine are registered by Watts as occurring in the flame of a 

 Bessemer converter ; or at least the lines he gives are so near 

 that we cannot doubt their identity. 



When we come to make a comparison with the spectrum 

 of the spark-discharge from a solution of ferric chloride, the 

 differences become more marked. Not only are there many 

 more lines in the spark-spectrum, but the relative intensities 

 of those lines which are common to both spark and explosion 

 are very different, and two of the iron lines seen in the ex- 

 plosions appear to be absent from the spark. The differences 

 between the spectrum of the spark taken from a liquid elec- 

 trode and that given by solid electrodes has usually been 

 attributed to the lower temperature of the former ; but the 

 absence from the former spectrum of the line at wave-length 

 4132, and the feebleness of the line at wave-length 4143, both 

 strong lines in the arc and in the explosions, as well .as in the 

 spark between solid electrodes, seem to indicate that the dif- 

 ferences of spark-spectra 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 assu- 

 ming 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 tempera- 

 ture 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. P. S. xxxiv. p. 119, and Proc. 

 Camb. Phil. Soc. iv. p. 256) w T e described the spectrum pro- 

 ceeding 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 between 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 ex- 

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



