CHAPTER XXXV 



BIOLOGICAL EFFECTS OF RADIATIONS 



(By Titus C. Evans, Colleges of Physicians, New York) 



The study of biological effects of radiations is making such rapid 

 progress that it is difficult to cover all of the important phases. 

 It is hoped that this chapter vrill serve as an outline of the subject, 

 and that the reader will study the references listed in the back of 

 the book for detailed information. A few points in physics and 

 chemistry must be reviewed to understand this subject. 



The Structure of the Atom 



The atom is thought to consist of (1) a central nucleus bearing a 

 positive charge and possessing nearly all of the mass of the atom, and 

 (2) one or more electrons (negatively charged particles of small 

 mass) revolving around the nucleus in definite orbits. The nucleus 

 contains protons (positively charged particles) and electrons. The 

 protons are in excess and this gives the nucleus its positive charge. 

 The number of electrons in the orbits is determined by the number 

 of protons in the nucleus in excess of the ones neutralized by the 

 nuclear electrons. Hydrogen, for example, consists of one proton in 

 the nucleus (no nuclear electrons) and one orbital electron. 



Atomic Weight. — Atomic weight is the total weight of the nu- 

 cleus. Heavy hydrogen has a nucleus twice the weight of hydrogen. 

 It contains, besides the proton, another particle of approximate 

 equal mass which is electrically neutral. This particle is termed a 

 neutron and is thought to be composed of a proton and an electron 

 in close association. 



The atomic number is related to the charge on the nucleus. Helium, 

 for example, contains two neutrons and two protons in its nucleus. 

 This gives it an atomic weight of four and an atomic number of 

 two. The atom is electrically neutral, so two orbital electrons are 

 present to balance the charge on the nucleus. The chemical prop- 

 erties of an element depend upon the number of orbital electrons 

 (atomic number). Those in the outermost shell are valence electrons 

 by which one atom may be joined to another. 



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