160 PHYSIOLOGY FOR NURSES 



found to weigh 15.88 times the volume of hydrogen. 

 Therefore, in the table of elements, hydrogen is repre- 

 sented by the letter "H" and its atomic weight by the 

 figure 1, Avhile oxygen is represented by the symbol or 

 letter "0" and its atomic weight by the figures 15.88. 

 In the same way uranium, the heaviest of all elements, 

 is represented by the symbol "U," and the atomic 

 weight by the figures 239.5 i.e., it is two hundred thirty- 

 nine and a half times heavier than hydrogen with 

 which it is compared. Atoms, of course, can not be 

 separated and weighed ; so the atomic weight is not an 

 actual but a relative weight, When more than one 

 atom occurs in any chemical combination, small figures 

 placed at the right of the symbol indicate the number 

 of atoms engaged in that combination. Thus H 2 means 

 two atoms of hydrogen combined with one of oxygen 

 to form the chemical combination of water. These three 

 atoms, thus united, form a molecule, and the molecular 

 weight of a substance is the sum of the atomic weights of 

 the elements entering into its combination. 



Examples: H..O = Hydrogen 2 atoms weighing 2 



Oxygen 1 atom " 15.88 



Molecular weight of water 17.88 



The formula of ammonia is H 3 N or 



Hydrogen 3 atoms weighing 3 

 Nitrogen 1 atom " 13.93 



Molecular weight of ammonia 16.93 



Hydrochloric acid is represented by the formula HC1 or 

 Hydrogen 1 atom weighing 1 

 Chlorine 1 " " 35.18 



Molecular weight of hydrochloric acid 36.18 



It will be noted that in these three combinations, 

 'hydrogen is represented by one, two, and three atoms 



