THE CENTURY'S PROGRESS IN CHEMISTRY 



heed till a decade or so later, when three new elements, 

 gallium, scandium, and germanium, were discovered, 

 which, on being analyzed, were quite unexpectedly 

 found to lit into three gaps which Mendeleeff had left 

 in his periodic scale. In effect, the periodic law had en- 

 abled Mendeleeff to predicate the existence of the new 

 elements years before they were discovered. Surely a 

 system that leads to such results is no mere vagary. So 

 very soon the periodic law took its place as one of the 

 most important generalizations of chemical science. 



This law of periodicity was put forward as an expres- 

 sion of observed relations independent of hypothesis; 

 but of course the theoretical bearings of these facts 

 could not be overlooked. As Professor J. H. Gladstone 

 has said, it forces upon us " the conviction that the ele- 

 ments are not separate bodies created without reference 

 to one another, but that they have been originally fash- 

 ioned, or have been built up, from one another, accord- 

 ing to some general plan." It is but a short step from 

 that proposition to the Proutian hypothesis. 



But the atomic weights are not alone in suggesting 

 the compound nature of the alleged elements. Evi- 

 dence of a totally different kind has contributed to the 

 same end, from a source that could hardly have been 

 imagined when the Proutian hypothesis was formulated, 

 through the addition of a novel weapon to the arma- 

 mentarium of the chemist the spectroscope. The per- 

 fection of this instrument, in the hands of two German 

 scientists, Gustav Robert Kirchhoff and Robert Wilhelm 

 Bunsen, came about through the investigation, towards 

 the middle of the century, of the meaning of the dark 

 lines which had been observed in the solar spectrum by 

 Fraunhofer as early as 1815, and by Wollaston a decade 



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