168 ANNUAL OF SCIENTIFIC DISCOVERY. 



matter, called the luminiferous ether. In the case of sound, a similar 

 phenomenon to the one under consideration is well known. We are 

 all acquainted with the principle of resonance ; if we sound a given 

 note in the neighborhood of a pianoforte, the string capable of giving 

 out the vibrations producing that note takes up the vibrations of the 

 voice, and we hear it answering the sound. The intenser vibrations 

 proceeding in one direction are absorbed by the string, and emitted 

 as waves of slighter intensity in every direction. 



Not only did Prof. Kirchhoff show experimentally that luminous 

 gases absorb the kind of light which they emit, by reversing the 

 spectra of several of the metals, but by help of theoretical consider- 

 ations he arrived at a very important general formula concerning the 

 emission and absorption of rays of heat and light, which includes 

 these phenomena as a particular case. The general law is called the 

 law of exchanges, and it asserts that the relation between the amount 

 of heat or of light which all bodies receive and emit is for a given 

 temperature constant. 



In order to determine and map the positions of the bright lines 

 produced by the electric spectra of the various metals, Kirchhoff 

 employed the dark lines in the solar spectrum as his guides. Much 

 to his astonishment, he observed that dark solar lines occur in posi- 

 tions coincident with those of all the bright iron lines. Exactly as 

 the sodium lines were identical in position with Fraunhofer's lines D, 

 for each of the iron lines (and Kirchhoff examined more than sixty) 

 a dark solar line was seen to correspond. Not only had each bright 

 iron line its dark representative in the solar spectrum, but the 

 breadth and degree of distinctness of the two sets of lines agreed in 

 the most perfect manner, the brightest iron lines corresponding to 

 the darkest solar lines. These coincidences cannot be the mere 

 effect of chance ; in other words, there must be some causal connec- 

 tion between these dark solar lines and the bright iron lines. That 

 this agreement between them cannot be simply fortuitous is proved 

 by Kirchhoff, who calculates from the number of the observed 

 coincidences, the distances between the several lines, and the degree 

 of exactitude with which each coincidence can be determined the 

 fraction representing the chance or probability that such a series of 

 coincidences should occur without the two sets of lines having any 

 common cause; this fraction he finds to be less than 1-1,000,000.000,- 

 000,000,000, or, in other words, it is practically certain that these 

 lines have a common cause. 



" Hence this coincidence," says Kirchhoff, " must be produced by 

 some cause, and a cause can be assigned which affords a perfect 

 explanation of the phenomenon. The observed phenomenon may bo. 

 explained by the supposition that the rays of light which form the 

 solar spectrum have passed through the vapor of iron, and have thus 

 suffered the absorption which the vapor of iron must exert. As this 

 is the only assignable cause of this coincidence, the supposition 

 appears to be a necessary one. These iron vapors might be con- 

 tained either in the atmosphere of the sun or in that of the earth. 

 But it is not easy to understand how our atmosphere can contain 

 such a quantity of iron vapor as would produce the very distinct 

 absorption lines which we see in the solar spectrum ; and this suppo- 



