90 Transactions of the 



diamond, fluorspar, &c., have the power of absorbing, when ex- 

 posed to a very bright light, a certain quantity of these rays, and 

 again of radiating or giving them out when placed in a dark room. 

 Becquerel has examined the spectra of various substances under 

 these circumstances, and has found that the light emitted by them 

 is peculiar in this way, that they yield broken spectra, or bands of 

 differently coloured rays. Alumina, for example, in this state of 

 phosphoresence emits a red light, and its spectrum exhibits four 

 bands between the lines C and H in the solar spectrum. 



There is another interesting property which certain bodies 

 possess, when exposed especially to the blue rays of the solar or 

 electric spectrum, namely, fluorescence. If a piece of Uranium 

 glass, for example, is held in the violet rays, it appears self- 

 luminous. This is caused by the transmutation of the rays into* 

 others, which are always of lower refrangibility. 



We come now to a most interesting and instructive part of this 

 subject — namely, solar and stellar chemistry. The electric spectrum 

 is made up of continuous bands, but in the solar dark lines are 

 observed. These are called Fraunhofer's lines, from the name of 

 the person who first mapped some of them out, and investigated 

 the subject. The mapping of these lines is of the highest impor- 

 tance, as, by their means, we can tell that some substances, as 

 iron, sodium, and magnesium, are present in the sun, and also in 

 the fixed stars. We do not know the composition of the planets, 

 or of the moon, because their light is reflected to us from the sun, 

 and consequently give the solar spectrum. But the fixed stars 

 give partly different lines ; hence we conclude that these lines are 

 not due to our atmosphere, but to the substances existing in the 

 sun and stars. 



So long ago as 1814, Fraunhofer concluded that these lines 

 were caused by some absorptive power exerted in the stars or 

 sun. 



And not only do these dark lines exist in the visible portions 

 of the spectrum, but they also occur in the portions which contain 

 the visible heating and chemically active rays. This can be shown 

 in the ultra-violet rays, but not in the ultra-red, by placing a 

 sensitized collodion plate in the former, when shaded spaces will 

 be produced, indicating the positions in which the intensity of 

 the rays is small. These are Fraunhofer's lines in the ultra-violet 

 sunlight. 



We now come to the important inquiry about the lines. How 

 are they formed ? Fraunhofer observed, many years ago, that the 

 dark lines D in the solar spectrum were coincident with the bright 

 sodium lines ; but it was not till the year 1859 that Kirchhoff 

 explained the phenomena thoroughly. He obtained a strong solar 



