254 fRA OMENTS OF SCIENCE. 



yield up more speedily their motion to the ether. Mix 

 oxygen and nitrogen mechanically, they absorb and radiate 

 a certain amount of heat. Cause these gases to combine 

 chemically and form nitrous oxide, both the absorption and 

 radiation are thereby augmented hundreds of times! 



In this way we look with the telescope of the intellect 

 into atomic systems, and obtain a conception of processes 

 which the eye of sense can never reach. But gases and 

 vapors possess a power of choice as to the rays which they 

 absorb. They single out certain groups of rays for de- 

 struction, and allow other groups to pass unharmed. This 

 is best illustrated by a famous experiment of Sir David 

 Brewster's, modified to suit present requirements. Into a 

 glass cylinder, with its ends stopped by disks of plate-glass, 

 a small quantity of nitrous acid gas is introduced, 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. Intro- 

 ducing 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 com- 

 parative impunity. 



Here also the principle of reciprocity, as regards radia- 

 tion 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 :i 

 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 volatilize 

 them. This is easily done by the voltaic current. A cap- 

 sule 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 volatil- 

 ized condition passes between them. The spectrum of this 



