229 



spectrum than the one above referred to ; we have, however, 

 noticed the formation of four new violet lines in the intense 

 strontium spectrum, and we nov\^ believe that the other two 

 lithium hues mentioned in our letter to the Philosophical 

 Magazine are caused by the presence of the most minute 

 trace of strontium floating in the atmosphere, and derived 

 from a previous experiment. We have convinced ourselves 

 by numerous observations that the currents of air caused by 

 the rapid passage of the electric spark between the electrodes 

 are sufficient to carry over to a second set of electrodes 

 placed at the distance of a few inches, a very perceptible 

 quantity of the materials undergoing volatilisation. The 

 greatest precautions must hence be taken when the spectra 

 of two metals have to be compared ; and no separate observa- 

 tions of the two spectra can be relied upon, unless one is 

 made a considerable space of time after the other, and unless 

 all the electrodes which have been once used are exchanged 

 for new ones. 



Kirchhoff, in his interesting Memoir on the Solar Spectrum 

 and the Spectra of the Chemical Elements,* noticed in the 

 case of the Calcium spectrum, that bright lines which were 

 invisible at the temperature of the coal gas flame became 

 visible when the temperature of the incandescent vapour 

 reached that of the intense electric spark. 



We have confirmed this observation of Kirchhofl''s, and 

 have extended it, inasmuch as we, in the first place, have 

 noticed that a similar change occurs in the spectra of 

 Strontium and Barium ; and, in the second place, that not 

 only nevf lines appear at the high temperature of the intense 

 spark, bat that the broad hands characteristic of the metal or 

 metallic compound at the low temperature of the flame or 

 weak spark, totally disappear at the higher temperature. 

 The new bright lines which supply the part of the broad 



* KirclihoiF on the Solar Spectrum, &c. Translated by H. E. Eoscoe. 

 JVIacMillanj Camljriclge. 1862. 



