84 PRACTICAL LESSONS IN SCIENCE. 



be crossed by great numbers of fine black lines, which for a long 

 time could not be explained. 



Each of the elements which make up our earth gives its own 

 peculiar spectrum ; as sodium gives two yellow lines, potassium 

 two red lines and one purple line, nitrogen many lines of different 

 colors, hydrogen two lines, etc. The colors are as variable as 

 those of the solar spectrum, and vary in number through an im- 

 mense range. For instance: Iron shows as many as 450 lines, 

 while sodium shows only two. On comparing the spectrum 

 of iron with the solar spectrum, the bright lines of iron were 

 found to correspond with certain black lines in the solar spec- 

 trum, and similar results were obtained by experimenting with 

 other substances. This coincidence called for explanation. Ex- 

 perimenting with sodium vapor it was found that light passing 

 through sodium vapor gave a spectrum that was perfect, except- 

 ing that the place of the yellow light was occupied by a dark band. 

 Similar results were obtained from experimenting with the vapor 

 of other substances. These facts led to the conclusion that vapors 

 absorb the kind of light which they emit, letting the other kinds 

 pass through. And these facts led to the further conclusion that 

 the sun consists of a molten mass surrounded by an atmosphere 

 of metallic vapors, which absorb the light from the molten metals 

 within, thus giving rise to the dark lines in the solar spectrum. 



An examination of the light of the fixed stars shows that 

 they are bodies like our sun, while the nebulae seem to be com- 

 posed entirely of gaseous matter. As the spectrum of the most 

 important terrestrial substances is known, the composition 

 of bodies may be determined by the spectroscope, and several 

 new substances have been discovered by its use. It has been 

 used to detect adulteration of liquids and diseased conditions 

 of the blood. By it iron, sodium, calcium, nickel, copper, 

 zinc, cobalt, manganese, aluminum, and other metals are shown 

 to exist in the sun. Hydrogen has been shown to exist in 

 hundreds of different stars. In the star Aldebaran, hydrogen, 

 iron, sodium, mercury, and five other elements common on the 

 earth are shown to exist, Some stars are so far from the earth 



