264 BIOLOGICAL EFFECTS OF RADIATION 



considerable extent but are not completely expelled, these molecules 

 with the displaced atoms may become activated so that they will react 

 with other molecules. The mechanism by which this electronic activa- 

 tion is transferred to energy of atomic displacement is not fully under- 

 stood. In some cases at least colUsions with other molecules are 

 necessary. In still other cases, collisions of these electronically excited 

 molecules with inert molecules are able to remove the energy of electronic 

 excitation in the form of increased kinetic energy and thus tend to prevent 

 chemical reaction. The energy of radiation must be at least as great as 

 the energy required for activation. It does not necessarily follow that 

 if the energy of the exciting light is as great as or greater than the energy 

 of activation that activation will automatically occur; for the energy of 

 activation may be immediately dissipated in other ways. 



It might be expected that infra-red radiation which is immediately 

 connected with the displacement of atoms in the molecule would lead 

 directly to chemical reaction. In this case it would not be necessary to 

 effect a transfer of electronic excitation into energy of, atomic displace- 

 ment. However, as seen from Table 1, the energy contained in the infra- 

 red radiation is less than that required for most ordinary chemical 

 reactions (perhaps 25,000 cal. for those occurring at room temperature). 



It is an observational fact that photochemical reactions are much 

 more often produced by ultra-violet light than by blue light and that 

 those produced by blue Ught are more common than those produced by 

 green or red Ught. Such a situation is to be expected from the fact that 

 the shorter wave-lengths of light contain greater amounts of energy as 

 shown in Table 1. It must be realized that the intensity of energy in 

 these photons is the significant thing rather than the total amount of 

 energy contained in a beam of light. The greater the intensity of the 

 beam of light the more quanta are available and the faster is a given 

 photochemical reaction, provided that the quanta are large enough. 

 However, if the individual photons in this particular beam of light con- 

 tain an insufficient amount of energy {i.e., wave-length is too long) no 

 photochemical reaction can occur at all, no matter how many quanta are 

 introduced. 



Chemiluminescence. — A few chemical reactions are known in which 

 light is emitted. A common example is the glow of yellow phosphorus 

 when exposed to air. The luminescence of the firefly, of decaying wood 

 and certain marine bacteria are all attributed to the emission of light 

 which accompanies the oxidation of luciferin, a substance which is found 

 to occur in these bacteria and in insects. Rapid and violent reactions 

 also are known to emit light. For example, when chlorine reacts with 

 metals, a flame is produced. This is not due to incandescence of small 

 particles, as in an ordinary flame (temperature radiation). Occasionally, 

 among the less violent reactions examples of chemiluminescence are 



