386 FRAGMENTS OF SCIENCE. 



to which are intercepted by the nitric gas, while the 

 light which falls upon the intervening spaces is per- 

 mitted to pass with comparative impunity. 



Here also the principle of reciprocity, as regards 

 radiation and absorption, holds good ; and could we, 

 without otherwise altering its physical character, render 

 that nitrous gas luminous, we should find that the very 

 rays which it absorbs are precisely those which it would 

 emit. When atmospheric air and other gases are brought 

 to a state of intense incandescence by the passage of an 

 electric spark, the spectra which we obtain from them 

 consist of a series of bright bands. But such spectra 

 are produced with the greatest brilliancy when, instead 

 of ordinary gases, we make use of metals heated so 

 highly as to volatilise them. This is easily done by 

 the voltaic current. A capsule of carbon filled with 

 mercury, which formed the positive electrode of the 

 electric lamp, has a carbon point brought down upon 

 it. On separating the one from the other, a brilliant 

 arc containing the mercury in a volatilised condition 

 passes between them. The spectrum of this arc is 

 not continuous like that of the solid carbon points, 

 but consists of a series of vivid bands, each corre- 

 sponding in colour to that particular portion of the 

 spectrum to which its rays belong. Copper gives its 

 system of bands; zinc gives its system; and brass, which 

 is an alloy of copper and zinc, gives a spectrum made 

 up of the bands belonging to both metals. 



Not only, however, when metals are united like zinc 

 and copper to form an alloy, is it possible to obtain 

 the bands which belong to them. No matter how 

 we may disguise the metal allowing it to unite with 

 oxygen to form an oxide, and this again with an acid to 

 form a salt ; if the heat applied be sufficiently intense, 

 the bands belonging to the metal reveal themselves with 



