254 FRAGMENTS 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 tbese gases to combine 
chemically and form nitrous oxide, both the absorption and 
radiation are thereby augmented hundreds of times! 
lu 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 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 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 
