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 now believe that the other two 
lithium lines 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 Kirchhoff ’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 new lines appear at the high temperature of the intense 
spark, but that the broad bands 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 
* Kirchhoff on the Solar Spectrum, &c. Translated by II. E. Koscoe. 
MacMillan, Cambridge. 1862, 
