16 BIOLOGICAL EFFECTS OF RADIATION 



nitrogen, as well as all gaseous chemical compounds) and will be treated 

 farther on. Line spectra are exhibited by monatomic gases, a class which 

 at ordinary temperatures comprises only the noble or inert gases (helium, 

 argon, neon, and the very rare gases krypton and xenon) and the vapors 

 of certain metals, among which mercury vapor is by far the easiest to 

 study, while those of the alkali metals are the next easiest. Many, 

 if not all, of these monatomic gases show spectra of both types, but 

 in such cases the band spectra are attributed to occasional molecules 

 mixed with the uncombined atoms. Some of the molecular gases show 

 line spectra when heated or exposed to electrical discharges, but these 

 are attributed to atoms which have been set free by the dissociation of 

 molecules. It happens that for the commonest gases, both monatomic 

 and molecular, most or all of the lines of the absorption spectrum are in 

 the ultra-violet, so that these gases seem to be perfectly transparent so 

 long as no account is taken of any but visible light. 



The line spectra, then, are the characteristic spectra of gases con- 

 sisting of single atoms. When such a gas is made more rarefied and 

 cooler, the lines of its absorption spectrum become sharper and narrower; 

 and it appears that we can approach quite closely to the ideal spectrum 

 of absolutely isolated and stationary atoms not colliding or interfering 

 with one another in any way. This limiting spectrum is the subject of 

 what next follows. 



Suppose that we have measured as many as possible of the absorption 

 lines of a given gas. Let Xt be the general symbol for the wave-length 

 of any one among them. I have said that in respect of absorption, 

 monochromatic light of wave-length X behaves like corpuscles of energy 

 hc/\ or hv. The existence of the absorption-line-spectrum thus signifies, 

 that atoms of the gas in question are able to absorb quantities of energy of 

 the amounts hc/\i or hui. The atoms are not able to absorb intermediate 

 amounts of energy, for there are gaps in the spectrum between the lines. 

 Atoms are capable of absorbing only certain specific discrete quantities 

 of energy. 



Another way of saying the same thing is: Atoms are capable of existing 

 only in certain specific discrete states, each distinguished by a characteristic 

 energy value; absorption of light by an atom entails the transfer or "transi- 

 tion" of the atom from one of these states to another, and consequently the 

 product of h by the frequency c/\i of any absorption line is the difference 

 between the energy values of two of the stationary states of the atom in question. 

 This is one of the fundamental principles discerned by Niels Bohr. 



In cool, rarefied, and unexcited monatomic gases the vast majority 

 of the atoms at any moment are in one particular state, the "normal 

 state" or "ground state." All of the absorption lines of such a gas 

 correspond to transitions out of this special state into various others, 



