"22 INORGANIC EVOLUTION. [CHAP. 



In 1869 I introduced into laboratory work the method adopted in 

 the case of the sun in the observatory ; that is, an image of each light 

 source experimented on was thrown on to the slit by a lens (Fig. 16), 

 so that the spectrum of each part of it could be observed, and some of 

 the results obtained by the new method were the following : 



FIG. 16. The method of throwing an image of the light source (in this case a 

 candle flame) on the slit plate of a laboratory spectroscope. 



The spectral lines obtained by using such a light source as the 

 electric arc or spark were of different lengths ; some appeared only in 

 the spectrum of the centre of the light source, others extended far 

 into the outer envelopes. This effect was best studied by throwing the 

 image of a horizontal arc or spark on a vertical slit. The lengths of 

 the lines photographed in the electric arc of many metallic elements, 

 were tabulated and published in 1873 and 1874. 



In Figs. 17 and 18 these so-called "long arid short lines" are 

 illustrated. In one case we deal with a mixture of the salts of calcium 

 and strontium, in the other with the metal sodium. The richness of the 

 lines in the spectrum of the core of the arc will be best gathered from 

 Fig. 17, the variations in the lengths of the lines from Fig. 18. 



Here then was the first glimpse of the idea that the complete 

 spectrum of a chemical element obtained at the highest temperature 

 might arise from the summation of two or more different line spectra, 

 produced at different degrees of temperature, and therefore bringing us 

 in presence of two or more molecular complexities ; that is, different 

 molecules broken up at different temperatures. So soon as experi- 



