166 Profs. Liveing and Dewar on the Spectral Lines 



cult to see whether any line was on the pointer; and the 

 observation could be repeated as many times as was desired 

 without any shift of the apparatus. Nine of the most conspi- 

 cuous green and yellowish-green lines in the flash were thus 

 identified with lines of iron. For the blue and violet we 

 adopted the photographic method as much le'ss trying to the 

 eyesight. Eight to twelve flashes were taken in succession 

 without any shift of the apparatus, so as to accumulate their 

 effects on the photographic plate. Eight flashes were found 

 enough in general to produce a good impression, and more 

 than twelve could not well be taken without turning out the 

 water wdiich accumulated in the tube, and cleaning the glass 

 which closed its end. After the flashes had been taken, but 

 without shifting the photographic plate, the slit of the spec- 

 troscope was partly covered, and the electric spark between 

 iron points passed in front of the slit. We had thus on the 

 plate the photograph of the flash as well as of the spark. 

 Fourteen more lines in the indigo and violet were thus iden- 

 tified with iron lines; and on extending the photographs into 

 the ultra-violet, and substituting quartz lenses and prisms for 

 the glass ones hitherto used, a much larger number of lines 

 were identified. There could be no doubt, then, that we had 

 iron vapour in the flash. We supposed that it must be de- 

 rived from dust of oxide shaken by the explosion off the sides 

 of the tube, and we had the tube bored out clean and bright 

 like a gun-barrel. This made no diminution in the brightness 

 or number of the lines; and we came to the conclusion that 

 the explosion detached particles of iron from the tube, and 

 converted them into vapour. This was confirmed by finding 

 that, however carefully the tube had been cleaned, the glass 

 ends always became clouded with a rusty deposit after ten or 

 twelve flashes. Altogether 68 lines of iron have been iden- 

 tified in the flash, of which about 40 lie in the ultra-violet 

 between H and 0. Only one iron line above has been 

 definitely identified, and that in only a few photographs. It 

 isT. 



As iron gave so many lines in the flash it was reasonable 

 to suppose that more volatile metals would give their lines 

 too. Linings of thin sheet copper, lead, cadmium, zinc, 

 aluminium, and tin were successively put into the tube, and 

 their effects on the flash observed. Copper gave one strong 

 line in the green (wave-length 5104*9), but no other line in 

 the visible part of the spectrum. In the ultra-violet two 

 strong lines between Q and R came out in the photographs, 

 frequently as reversed lines. Some of the photographs showed 

 also something of the shaded bands in the blue which are 



