4QO HISTORY OF SCIENCE. 



other new metals to our list of chemical elements. Thus, in 1864, 

 Professors Reich and Richter, of Freiberg, in Saxony, found a metal, 

 the spectrum of which is distinguished by a blue and a purple line. 



The method of obtaining spectra which has just been described 

 answers perfectly for the compounds of the alkalies and the earths, viz., 

 for the detection of potassium, sodium, lithium, rubidium, caesium, 

 thallium, indium, calcium, barium, and strontium ; but compounds of 

 the other metals for the most part fail to exhibit any characteristic 

 spectrum when their compounds are simply brought into the flame of a 

 Bunsen's burner. It will be remembered, however, that the spectrum of 

 the electric spark was examined for the first time by Wollaston (page 

 4 7 9), but the inquiries which his observation suggested were not pursued 

 at the time. It was very naturally supposed that the luminous appear- 

 ance was electricity itself, until Faraday conclusively proved that elec- 

 tric sparks are attended with a transference of some portion of metals 

 between which they pass ; and that, in fact, the luminous effect is due 

 to the intense ignition of small portions of the metals or other sub- 

 stances. These deductions were completely confirmed by Wheatstone, 

 who, in 1835, announced that the spectra of electric sparks were dif- 

 ferent as they were taken between different metals, and he considered 

 that the spectra were due not to the combustion of the metals but to 

 their volatilization. Indeed, Fox-Talbot had observed the previous 

 year certain bright lines in spectra yielded by deflagrating gold and 

 copper by the passage of a current from a powerful galvanic battery. 



Twenty years later the spectrum lines yielded by electric sparks taken 

 between different metals were undergoing very careful and laborious 

 investigations in the hands of a number of eminent experimentalists. 

 The greater part of the progress made in this direction is due to the 

 labours of the Swedish physicist Angstrom, to those of Kirchhoff in 

 Germany, and to those of Mr. Huggins in England. These observers 

 found that the lines peculiar to the spark-spectrum of each metal were 

 very numerous. Sets of maps and tables of the positions of the lines 

 were prepared by each of the philosophers we have named and by 

 others. The amount of labour incurred in these undertakings will be 

 better understood when it is explained that instead of the comparatively 

 few lines which a single-prism spectroscope exhibits, the spark-spectra 

 visible with the instruments in which a train of prisms is used instead 

 of a single one, show lines that in some cases may be numbered by 

 hundreds. Fig. 224 shows the arrangement of a spectroscope with 

 nine prisms, which was made for Mr. Gassiot by Mr. Browning. This 

 number has been exceeded in other instruments, and the ingenious 

 device has also been adopted of causing the rays to be reflected so as 

 twice to traverse the train of prisms. Although some thousands of lines 

 of spectra have been mapped, it is probable that the number of visible 

 lines may increase with every increase in the power of the instrument. 



It was soon observed that the spectrum lines given by any substance 



