COLOR AND CHEMICAL CONSTITUTION 117 



Fraunhofer had noticed that the yellow spectrum line from com- 

 mon salt, when fed into a spirit lamp, was identical in position with 

 the D-line of the solar spectrum. But though the formation of these 

 discontinuous spectra from various salts in a flame was generally 

 known, it was not until 1859 that the presence of the Fraunhofer lines 

 in the solar spectrum was clearly explained by Kirchhoff, who deduced 

 the following law : " The relation between the powers of emission and 

 the powers of absorption for rays of the same wave-length is constant 

 for all bodies at the same temperature." Thus the particles of a sub- 

 stance under the excitement of some outside force are thrown into a 

 state of vibration which is dependent upon the chemical nature of the 

 substance itself. This vibratory motion gives rise to waves in the ether 

 and we have the phenomenon of emission. Again the particles of a 

 substance are most responsive to these same characteristic vibrations 

 and will absorb them whenever present, just as, by analogy, the strings 

 of a piano pick up sound waves of the exact period in which they 

 vibrate when these waves are set in motion by other musical instru- 

 ments in the neighborhood. Kirchhoff explained the solar spectrum 

 as one produced by a strong white light from an interior sphere passing 

 through a concentric layer of vapors of many substances, each of which 

 absorbs those particular rays that correspond to their own periods of 

 vibration. The light, thus deprived of many definite rays, indicated 

 their absence when its spectrum was cast upon a screen by the appear- 

 ance of dark lines — the images of the slit through which the light 

 passed — corresponding always to the wave-lengths absorbed. It must 

 not be assumed that these lines of absorption are regions of total dark- 

 ness. The particles set in vibration by the rays absorbed will naturally 

 give out some light of this same vibration period, but the light emitted 

 is so small in comparison with the rays from the original source which 

 pass through unmolested that the image cast upon the screen will give 

 the appearance of almost total darkness. 



Now when a substance is yellow in color we can readily ascertain 

 that the spectrum of the light it reflects is lacking in a number of 

 rays of various wave-lengths. These rays correspond to the comple- 

 ment of the color reflected, and in the case of a yellow substance 

 belong to that magnitude found in the blue end of the spectrum. If 

 no wave-lengths of the visible spectrum had been absorbed, we should 

 have had the continuous spectrum of white light in the light reflected, 

 i. e., the body itself would not be colored. Colored substances, there- 

 fore, absorb the rays of their complementary colors and, consequently, 

 when white light is transmitted through them their spectra will indi- 

 cate the regions of this absorption by dark bands of varying intensity. 

 The absorption spectrum coincides always with the spectrum obtained 

 from the reflected light. 



