92 THE CHEMISTRY OF THE SUN. [CHAP. 



but he remarks that the spectroscope itself should be capable of 

 settling this question of impurities an opinion which we shall 

 see subsequently has been in part at least verified. 



The difficulty Thalen experienced in this question of impuri- 

 ties will be seen from his remarks on titanium. Concerning 

 this element, he states that the faintness and transitory cha- 

 racter of its lines rendered it difficult to determine any solar 

 coincidences, but he nevertheless found some such coinci- 

 dences in the green. 



Subsequently Angstrom and Thalen observed these lines in 

 the electric arc taken between poles of charcoal soaked in a 

 solution of calcic chloride, but they were not seen in the spark 

 between metallic electrodes moistened with a solution of calcic 

 chloride. 



Thalen then prepared some chemically pure bichloride of 

 titanium, and satisfied himself spectroscopically, by the absence 

 of the calcium lines, that it contained no trace of calcium, 

 while from the presence of the green lines above mentioned he 

 concluded that those lines are certainly due to titanium and 

 not to calcium ; and, further, that the carbons employed as elec- 

 trodes in the voltaic arc contained an impurity of titanium. 



From the number of fine lines of titanium concident with solar 

 lines Thalen concluded the existence of titanium in the sun. 



In the case of iron and other bodies he notes that there is an 

 agreement to some extent between the intensities of the solar 

 and metallic lines, but that in the case of titanium this is less 

 apparent. 



KirchhofF s and Angstrom's maps are in all our laboratories, 

 nnd there is a very considerable difference between them. This 

 difference arises from the fact that whereas KirchhofF used an 

 induction coil and spark, Angstrom varied his experimental 

 method by placing no longer a spark, but the electric arc in 

 front of the slit of his instrument. In this case, therefore, he 

 was determining the spectrum which was produced at the 



