286 CHEMISTEY. 



delicacy. It follows from what we have told you in the 

 preceding sections that any chemical substance capable 

 of being converted into an incandescent vapor by the heat 

 of a Bunsen burner must give out light of a particular 

 degree of ref rangibility ; and consequently any one look- 

 ing through the telescope, B, will see a pictorial image in 

 brilliant colors characteristic of that particular substance. 

 Practically this is done as follows : Dip a small platinum 

 wire into the material you wish to examine, insert the sub- 

 stance into the flame (which, being non-luminous, gives no 

 spectrum), and place your eye at B. Now it is found that 

 only a certain number of chemical substances are capable 

 of being volatilized in the heat of a Bunsen burner or of 

 an alcohol lamp ; these are the salts of the alkalies, many 

 of the salts of the alkaline earths, besides some other bodies 

 not classifiable ; or, stating it differently, spectrum analysis, 

 under the circumstances described, enables the chemist to 

 detect sodium potassium (lithium, caesium, rubidium), calci- 

 um, strontium, barium, copper, boracic acid, and some other 

 bodies. The spectra seen are shown in Fig. 107 (p. 287), 

 and in the frontispiece to this work. 



The first band represents the spectrum of the sun, the 

 vertical black lines in which you may for the present dis- 

 regard. 



Sodium gives a single yellow line or band, occurring 

 at a on the scale ; potassium gives a red line at the right 

 end of the spectrum, a blue one at the extreme left, and 

 a long luminous band between. Barium gives a large 

 number of lines and bands, several red, orange, yellow, and 

 four very bright green ones. These lines and bands al- 

 ways occur at the same point on the scale of the same 

 spectroscope ; the scales of various instruments vary, but 

 the positions of lines can be compared by preparing maps 

 of the various spectra referred to a constant scale. Thus 



