Analysis by Spectrum-observations. 93 



In order to prove still more conclusively that each of the 

 above-mentioned metals always produces the same bright lines 

 in the spectrum, we have compared the spectra represented in 

 the Plate with those produced when the electric spark passes 

 between electrodes made of these metals. 



Small pieces of sodium, potassium, lithium, strontium, and 

 calcium were fastened to fine platinum wires and melted two 

 by two into glass tubes, so that the pieces of metal were sepa- 

 rated by about 1 to 2 millims., and the platinum wires were 

 melted through the sides of the glass tubes. Each of these 

 tubes was placed in front of the spectrum-instrument, and 

 by means of a RuhmkorfPs induction apparatus, sparks were 

 allowed to pass between the pieces of metal inside the tube ; 

 the spectrum thus produced was then compared with that given 

 by a gas-flame in which the chloride of the metal was brought. 

 The flame was placed behind the glass tube. By alternately 

 bringing the induction apparatus into and out of action, it was 

 easy, without measuring, to convince ourselves that in the 

 brilliant spectrum of the electric spark, the bright lines of the 

 flame-spectrum were present in their right position. Besides 

 these lines, other bright ones appeared in the electric-spark 

 spectrum ; some of these were produced by foreign metals present 

 in the electrodes, others arose from nitrogen, which filled the 

 tubes after the oxygen had combined with a portion of the 

 electrodes*. 



From these facts it appears certain that the appearance of 

 the bright lines represented in the spectra on the Plate may be 

 regarded as absolute proof of the presence of the particular 

 metal. They serve as reactions by means of which these bodies 

 may be recognized with more certainty, greater quickness, and 

 in far smaller quantities than can be done by help of any other 

 known analytical method. 



The spectra drawn on the Plate represent those seen when 

 the slit was of such a width that only the most conspicuous of 

 the lines of the solar spectrum were visible, the magnifying 

 power of the telescope C being a fourfold one, and the light of a 

 moderate degree of intensity. These circumstances seem to us 

 to be the most advantageous when it is required to make a 

 chemical analysis by means of spectrum-observations. The 



* On employing on one occasion with strontium-electrodes a tube filled 

 with hydrogen instead of nitrogen, the stream of sparks changed rapidly 

 into a continuous arc of light, whilst a grey pellicle covered the inside of 

 the tube. The tube was opened under rock-oil, when it was found that it 

 was empty, the hydrogen having disappeared. This gas appears, at the 

 enormous temperature of the electric spark, to have decomposed the oxide 

 of strontium which was not completely removed from the metal. 



