SCIENTIFIC RECREATIONS. 



suppose we shut out the daylight and substitute an artificial light. If we 

 use a lamp burning alcohol with salt (chloride of sodium), the spectrum will 

 only consist of two yellow bands, all the other colours being absent. With 

 lithium we obtain only two, one orange and one red. From this we deduce 

 the fact that different substances when burning produce different spectra ; 

 and although a solid may (and platinum will) give all seven colours in its 

 spectrum, others, as we have seen, will only give us a few, the portion of 

 the spectrum between the colours being black. Others are continuous, and 

 transversed by "lines" or narrow spaces devoid of light ; such is the spectrum 

 of the sun, and by careful and attentive calculation and observation we can 

 get an approximate idea of the matter surrounding the heavenly bodies. ^ 



We have said there are lines crossing the spectrum transversely; these 



Fig. 150. The Spectroscope. 



are called Fraunhofer's lines, after the philosopher who studied them ; they 

 were, however, discovered by Wollaston. These lines are caused by light 

 from the lower portion of the sun passing through the metallic vapours 

 surrounding the orb in a state of incandescence, such as sodium, iron, etc. 

 One of Fraunhofer's lines, a black double line known as D in the yellow 

 portion of the spectrum, was known to occupy the same place as a certain 

 luminous line produced by sodium compounds in the flame of a spirit lamp. 

 This gave rise to much consideration, and at length Kirchkoff proved that 

 the sodium vapour which gives out yellow light can also absorb that light ; 

 and this fact, viz, that every substance, which at a certain temperature emits 

 light of a certain refrangibility, possesses at that temperature the power to 

 absorb that same light So the black lines are now considered the reversal 

 of luminous lines due to the incandescent vapours by which the sun is 

 surrounded. Thus the presence of an element can be found from black or 

 'uminous lines, so the existence of terrestrial elements in celestial bodies has 



