SECT. iv. SPECTRUM OF LIGHTNING. 147 



lines, hitherto only brought out by an intense electric 

 spark. 1 



M. Bunsen produced a beautiful effect by vaporizing 

 a mixture of equal parts of the chlorides of sodium, 

 potassium, lithium, calcium, strontium, and barium, 

 and passing the light through the slit of his apparatus. 

 For on looking through the telescope the spectrum of 

 each substance with its characteristic coloured lines in 

 all their brilliancy came successively into view, and 

 gradually faded away as each substance was volatilized 

 and driven off. The sequence showed the time required 

 to vaporize each metal, and by spectrum analysis each 

 metal could be recognized, although the mixture only 

 contained the T -fa -$ part of a grain of each chloride. 



The position, colour, and nature of the bright lines on 

 the spectra of more than thirty metals have been deter- 

 mined, besides those of the elementary gases and that 

 of the electric spark. To these M. Louis Grandeau has 

 added the spectrum of lightning. By a particular ar- 

 rangement the light passed at once through the slit in 

 the instrument, and a glass tube containing nitrogen 

 and the vapour of water. The general appearance of 

 the lightning spectrum at first recalled that of the 

 electric spark, but on a closer examination, H. Grandeau 

 noticed in the spectrum of almost every flash the coin- 

 cidence of a certain number of the rays of the lightning 

 spectrum with those of the spectra of nitrogen and 

 hydrogen. M. Grandeau remarks that this result is 

 not surprising, since all admit the production of ammonia 

 and nitric acid under the influence of electrical dis- 

 charges. Besides the rays of nitrogen and hydrogen, 

 the lightning spectrum contains the ubiquitous yellow 

 ray of sodium. 



Fraunhofer had noticed a coincidence between the 



1 ' On the Means of Increasing the Intensity of Metallic Spectra.' By 

 Mr. W. Crookes. 



L2 



