SOME INTERACTIONS WITH LIVING MATTER 93 



and damage the color pigment molecules. Snow-blindness and "whiteouts" 

 are the result. Further, ultraviolet has been attributed in some cases to 

 promoting the growth of cataracts and photothalamia, or inflammation of 

 the cornea. However, ordinary window glass absorbs all the dangerous 

 ultraviolet, and colored inorganic materials can be added to filter out (or 

 absorb) any undesired range of wave lengths. Therefore, protection is no 

 problem, if properly sought. 



Ultraviolet light has a lethal effect on primitive animal and plant life. 

 This fact is used to good advantage in destroying the bacteria, eschenchia coli 

 and bacteria coli, in foods or in our water supply. Each of these is killed by 

 about 14 x 10" 6 ergs per bacterium. Among the abnormalities successfully 

 treated with ultraviolet light are conjunctivitis, fibrosis, acne, and surface in- 

 fections of various kinds. Certain heavy metals (calcium, gold, silver, etc.) 

 and certain highly absorptive molecules (methylene blue, quinine, etc.) 

 sometimes increase the therapeutic value of the ultraviolet irradiation. 



The shortest-wave, vacuum-ultraviolet radiation overlaps the X-ray re- 

 gion. The principle difference between the two regions in the present classi- 

 fication is whether ionization and bond rupture is the exception (ultraviolet) 

 or the rule (X and gamma). The vacuum-ultraviolet will be discussed im- 

 plicitly in the next section, for the differences between it and the X ray are 

 of degree rather than of kind. 



Ionizing Radiations (Mainly X and Gamma) 



Principles 



The only distinction between the radiations more and less energetic than 

 that with a wavelength about 2000 A is one of excitation vs ionization. That 

 is, at wavelength X greater than about 2000 A, excitation of electrons of the 

 electron cloud takes place as the rule, and ionization takes place only in 

 special circumstances; while at X less than about 2000 A the electrons can be 

 knocked right out of the atom by the absorbed photon. As X decreases, the 

 loosely held orbital electrons are the first to go, followed by the subshell elec- 

 trons, and as X — » 1 A (X-ray region) the tightly bound K-shell electrons 

 can be ejected. 



A simple calculation will make this important point clear. It takes an in- 

 put, w, of ~230 kcal to make 1 mole of ions out of 1 mole of atoms, i.e., 

 10 ev to make an ion out of an atom. (This is the energy carried by each 

 photon of em radiation of wavelength 1200 A.) Now the gamma radiation 

 of the radioactive isotope of cobalt of atomic weight 60 (referred to the hy- 

 drogen atom as 1), Co 60 , used in deep radiation therapy for cancer, has an 

 energy of about one million electron volts (1 mev/photon). Therefore, each 

 photon would leave a wake of about 1 6 / 1 = 10 5 pairs of ions (or molecules 

 which have been ionized) before it loses all its energy. 



