552 On the Spectra of Erbia and some other Earths. [June 16, 



you kindly gave me was attached to a platinum loop with syrupy phos- 

 phoric acid as usual, and decomposition of the salt effected in the plain 

 hydrogen flame. After heating for a short time in this way, the chief 

 green line of erbia became visible, but seen upon the continuous spectrum. 

 Oxygen was now turned slowly into the flame. As the temperature rose, 

 two of the other bright lines of the earth were seen. The best observa- 

 tions were made when the oxyhydrogen flame had hydrogen in excess, 

 and the erbia was kept in such a position that it was very strongly ignited. 

 The erbia lines were most distinctly seen when the slit of the spectroscope 

 took in the light from the extreme edge of the incandescent solid. When 

 the bright lines were best observed, the continuous spectrum was relativelv 

 faint. Again, when the slit was made to cut the edge of the ignited bead 

 of the earth, the strong green line of erbia was seen to extend to a very 

 small but appreciable distance above or below (as the case might be) the 

 continuous spectrum. I could only observe this for the strong line. I 

 failed to get any trace of lines in the spectrum of the flame beyond the in- 

 candescent erbia. 



" The erbia was next heated in the oxyhydrogen flame to the maximum 

 temperature that the wire would bear for three and a half hours, but the 

 green line was seen to be just as strongly marked at the end as at the 

 beginning of the experiment. The bulk of the erbia was so much reduced 

 by this treatment, that I have now scarcely a trace left. 



"From the results of these experiments, I think we must conclude (1) 

 that the light emitted by incandescent erbia is derived chiefly from the 

 ignited solid, but that the bright lines observed in its spectrum have as 

 their source a luminous vapour of extremely low tension at even the 

 highest temperature of the oxyhydrogen flame ; (2) that this interrupted 

 spectrum belongs either to erbium or to its oxide. 



" If these conclusions are true, it follows that erbia is not an exception 

 to the ordinary law. 



" It would appear that in these experiments three substances have been 

 employed, varying in their degree of volatility. At the temperature of the 

 oxyhydrogen flame magnesia appears to be less volatile than lime ; but I am 

 in doubt what relative volatility to assign to erbia, since its spectrum of 

 bright lines can be seen when the earth is heated in the plain hydrogen 

 flame, and yet at the much higher temperature of the oxyhydrogen jet the 

 volume of luminous vapour does not appear to materially increase. 



" Finally, we have yet to learn whether or not in all these cases reduction 

 of the oxide precedes volatilization ; if reduction takes place, the luminous 

 vapour must be that of the metal. The settlement of this question 

 would no doubt be very difficult. But I rather incline to the view that the 

 vapour whose spectrum is obtained on igniting these earths is that of the 

 metal; for I find that the bright lines are most easily observed when 

 hydrogen is present in excess in the oxyhydrogen flame. Moreover, the 

 actual amount of matter volatilized on very prolonged heating is really very 



