334 FKAGMENTS OF SCIENCE. 



the splendid and continuous spectrum becomes instant- 

 ly furrowed by numerous dark bands, the rays answer- 

 ing to which are intercepted by the nitric gas, while 

 the light which falls upon the intervening spaces is 

 permitted 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, ren- 

 der 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 ob- 

 tain from them consist of a series of bright bands. 

 But such spectra are produced with the greatest bril- 

 liancy 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 car- 

 bon 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 spec- 

 trum of this arc is not continuous like that of the solid 

 carbon points, but consists of a series of vivid bands, 

 each corresponding 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 



