256 BIOLOGICAL EFFECTS OF RADIATION 



of the dissociation process from infra-red absorption and from band 

 spectra because they are intimately connected with the displacement of 

 atoms within the molecule. 



The energy required to break chemical bonds and thus make possible 

 chemical reaction amounts to many thousand calories per gram mole- 

 cule of material. Reactions which proceed with measurably slow 

 velocities at room temperature usually require in the neighborhood of 

 25,000 cal. and the most difficult reactions, those which go only at very 

 high temperatures, require up to 100,000 cal. and more. When* a new 

 product is formed and new chemical bonds result, energy is given out 

 corresponding to the energy which is absorbed in the breaking of these 

 bonds. It is clear that the energy of activation is partially offset by the 

 energy of combination. The heat of reaction, then, is the difference. 

 This heat of reaction must be less than the energy of activation and it 

 usually happens that the energy released by the products is so much 

 greater than the energy required to activate the reactants that heat is 

 evolved in the chemical reaction. It is a common observation that 

 exothermic reactions are much more common than endothermic reactions 

 at room temperatures. 



Activation of Molecules. — The thermal activation of molecules has just 

 been discussed. There are various other ways in which molecules can 

 be activated. Collisions with electrons, for example, will provide suffi- 

 cient energy to produce the necessary activation of the molecule. The 

 energy of the collision depends on the velocity of the electron, and this can 

 be controlled by the applied voltage. Important calculations can be 

 made from the ionization potential and the resonance potential which 

 are determined by finding the applied voltage at which the electrons are 

 absorbed by the molecules. Activation of molecules may be produced 

 also by collision with the alpha particles emitted by radioactive sub- 

 stances. An alpha particle with a mass four times that of the hydro- 

 gen atom traveling with one-tenth the velocity of light contains sufficient 

 energy to ionize and activate 100,000 molecules. Chemical activation 

 may be effected also by collisions of molecules with units of radiation, 

 called photons. These photons are presumably distributed in a random 

 manner in a beam of radiation and each photon contains a quantum of 

 energy, the value (in ergs) depending on the wave-length. This collision 

 process between molecules and photons forms the foundation of 

 photochemistry. 



Again, molecules can be activated by collisions with other molecules 

 which have already become activated or excited. For example, a 

 mercury atom which has absorbed ultra-violet light contains an abnor- 

 mal amount of energy, and when this excited mercury atom collides 

 with a molecule of ammonia, for example, the ammonia molecule is 

 decomposed. 



