352 FRAGMENTS OF SCIENCE 



duced; the presence of the gas being indicated by its rich 

 brown color. The beam from an electric lamp being sent 

 through two prisms of bisulphide of carbon, a spectrum 

 seven feet long and eighteen inches wide is cast upon the 

 screen. Introducing the cylinder containing the nitrous 

 acid into the path of the beam as it issues from the lamp, 

 the splendid and continuous spectrum becomes instantly 

 furrowed by numerous dark bands, the rays answering 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 radi- 

 ation and absorption, holds good; and could we, without 

 otherwise altering its physical character, render that ni- 

 trous 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 elec- 

 tric spark, the spectra which we obtain from them consist 

 of a series of bright bands. But such spectra are pro- 

 duced with the greatest brilliancy when, instead of ordi- 

 nary gases, we make use of metals heated so highly as to 

 volatilize them. This is easily done by the voltaic cur- 

 rent. 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 volatilized 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 color to that particular portion of 

 the spectrum to which its rays belong. Copper gives its 



