3SO GREAT MEN OF SCIENCE 



elements by means of the spectroscope; in this way he dis- 

 covered thallium, in 1861. Inasmuch as in the same way 

 elements which only occurred in traces in substances found 

 on the earth's crust were gradually brought to light, the 

 possibility then appeared of ordering all the known elements 

 according to their atomic weight in such a manner that series 

 of elements were found all having similar properties. This 

 'natural system of the elements' then immediately allowed 

 the gaps to be seen by inspection, in which elements still 

 undiscovered belong, and the number of these gaps is 

 already approaching zero. The study of fluorescence by 

 means of cathode rays, which was begun by Crookes, has 

 shown itself in its modern form, in which invisible and high 

 frequency fluorescence is used, to be a particularly effective 

 method for the discovery of the last and still missing ele- 

 ments. The spectrum of this fluorescence, produced by 

 means of crystal gratings, shows not merely the presence of 

 new elements by new lines, but also betrays, by the position 

 of the lines, the atomic number of the new element in the 

 periodic system, whereby we are able to calculate directly the 

 approximate value of the atomic weight, and also to derive 

 many indications of the chemical behaviour of the element 

 in question. 



JOSEF STEFAN {1835-1893) 

 LUDWIG BOLTZMANN {1844-1906) 



These two investigators made such important developments 

 in the views initiated by Kirchhoff concerning the light and 

 heat radiation of hot bodies, that their work served as the 

 starting point from which the great bulk of our knowledge in 

 this field up to to-day has been acquired. Kirchhoff had, by 

 means of his law, established a definite connection between 

 the emission and absorption of radiation. This already 



