Chap. 2 LIFE IS A CONCERN OF MATTER AND ENERGY 15 



hesion of the nucleus. Those that do not readily change are called stable 

 isotopes; the radioactive isotopes give off nuclear energy. Isotopes have been 

 detected in nature and many radioactive ones have been made in laboratories. 

 The separation of isotopes is a means of exploring changes that take place 

 within the nuclei of atoms. One of the problems in dealing with isotopes is to 

 separate out the kind which is to be used. In some cases this is easy; in others 

 it is extremely difficult. In the distillation of water the vapor which first con- 

 denses is water containing the light isotope of hydrogen. Later the heavy water 



Oxygen 



Carbon 



Hydrogen 



Nitrogen 



Misc. 



Water 



Proteins 



Carbohydrates, 

 Lipoids, Minerals 



Fig. 2.5. Top, Percentages of different kinds of atoms in the human body. In- 

 cluded under miscellaneous are, in order of decreasing amounts, calcium, phos- 

 phorus, potassium, sodium, sulfur, chlorine, magnesium, and iron. Bottom, Percent- 

 ages of different kinds of molecules in the human body. (Modified from Moment: 

 General Biology. New York, Appleton-Century-Crofts, 1950.) 



