From Newton to Kirchojf. 



209 



B. Each element invariably gives the same set when 

 burned under the same conditions of temperature and 

 pressure. 



C. The lines from no two elements are ever alike in po- 

 sition, and seldom in number or color. 



Upon these principles are based the spectrum tests, valued 

 so highly by chemical analysts. The discovery of new 

 elements, caesium and rubidium, by M. Bunsen ; thallium, 

 by Mr. Crooks, and indium by Drs. Eeich andBichter, 

 substances of which nature seems to have been so chary in 

 her distribution that all other tests had failed to detect 

 them, attests the value and the delicacy of this method of 

 analysis. Of the exceeding delicacy of the test, its appli- 

 cation to the metal sodium furnishes an illustration. The 

 spectrum of this substance consists of two very bright 

 yellow lines, separated by a distance visible only when 

 magnified by a good instrument. But notice : in the invisi- 

 ble dust that floats at all times in the air, there is enough 

 of a compound of this metal constantly in contact with a 

 gas flame to write in a spectroscope the story of its exist- 

 ence by a flash of light. 



Let us not, however, linger over the delicacy and value 

 of spectrum analysis as a method of chemical research. 

 Its application to the study of the composition of terrestrial 

 matter was but an incident, albeit an important one, in the 

 process of its development. 



During all the time when the beautiful spectra of arti- 

 ficial light were attracting so much attention, the study 

 of the Fraunhofer lines was being pursued with undi- 

 minished zeal. Indeed, as acquaintance with the former 

 became more intimate, interest in the latter was intensified. 

 There was seen to be analogy enough between the two 

 sets of phenomena to excite a vague suspicion that some 

 chain of relationship linked them to a common parentage. 

 Fraunhofer had, himself, noticed that a prominent dark 



Trans. vii.~\ 27 



