354 Dynamic TJieory. 



existing in an incandescent vaporous state in the atmosphere of the sun. 

 If any of these bodies heated to an incandescent vaporous state is al- 

 lowed to form its own spectrum, such spectrum will consist merely of 

 certain bright or colored cross-bands disconnected from each other in- 

 stead of the continuous colored spectrum 

 formed by sunlight or by an incandes- 

 cent solid or liquid body. The number 

 and position of the colored cross-bands 

 of these discontinuous spectra vary 

 with the bodies forming them, no two 

 having lines in precisely the same 

 position on the spectrum. But the 

 position of all these lines in the 

 bright spectra of the incandescent vapor- 

 ous bodies forming them is identical 



FIG. 159. Diagram showing Spectrum of Light and Heat rays of the Sun. 



All lying to the left of 80 represents the invisible heat rays, that to the right of 80 in- 

 dicates the visible part of the spectrum. The lines marked A and B are Fraunhofer's 

 lines in the red part of the spectrum. D is the sodium line in the orange part of the spec- 

 trum. F is a hydrogen line in the green. H is in the violet. To the right of H and the 

 left of A the spectrum fadesinto invisibility. The figures represent the length of the waves 

 of the radiant energy, and stand for so many hundred thousandths of a millimeter. The 

 dotted line X represents the surface of the earth and foot of mountains ; Y is the top of 

 the mountains, and Z the top of the atmosphere. The lines show the kinds of light and 

 heat which predominate in the respective localities. Blue light is the prevailing light at 

 the top of the atmosphere. ( After Prof. S. P. Langley. ) 



with the position of the dark lines formed on the continuous spectrum 

 of an incandescent solid or liquid when its light passes through such in- 

 candescenf vaporous bodies. The whole number of these lines in the 

 solar spectrum in the case of hydrogen is four, of calcium 75, of nickel 

 33, of manganese 57, of iron 600, &c. l Thus it appears 1st, that an 

 incandescent solid or liquid body emits waves of all the lengths in the 

 44th octave, that is, from 435 to 871 trillions per second. (2d.) That 

 an incandescent vapor or gas emits waves of certain lengths correspond- 

 ing with only a portion of those emitted by the solid, some of one velocity 

 and some of another. ( 3d. ) That when the waves from the solid are 

 absorbed by the vapor, they are reduced to stillness or darkness, their 

 motion being transferred to other particles in the same body having a 

 slower rate of vibration or fundamental tone whereby such particles are 

 caused to vibrate in greater amplitude, thus increasing the degree of 

 heat. A vapor when non-luminous has just the same effect as when it is 

 incandescent, quenching light of the same color that it emits when hot. 

 1 Lockyer 246. 



