234 ANIMAL BIOLOGY 



in a convenient form chemists have given the elements 

 certain symbols, H for hydrogen, O for oxygen, C for 

 carbon, N for nitrogen, S for sulphur, K for potassium 

 (kalium), Na for sodium (natrium), etc. The chemical 

 composition of bodies may be indicated by groups of 

 symbols representing their constituent elements: salt, 

 NaCl; water H 2 O; carbon dioxide, C0 2 ; sulphuric acid, 

 H2SO4; etc. Each molecule of common salt is supposed 

 to contain one atom of sodium and one atom of chlorine; 

 each molecule of water two atoms of hydrogen to one of 

 oxygen; and each molecule of sulphuric acid, two atoms 

 of hydrogen, one of sulphur and four of oxygen. 



Chemical changes or reactions are expressed in the form 

 of an equation; thus the formation of water by the ad- 

 dition of oxygen and hydrogen is indicated by 2H + O = 

 H 2 O and the decomposition of calcium carbonate by heat, 



CaCO 3 = CaO + CO 2 



Calcium Calcium Carbon 

 carbonate oxide dioxide 



Besides the chemical changes resulting from the simple 

 combination or dissociation of elements as in the two 

 illustrations just given, we may have, in bringing together 

 two compounds, an exchange of certain of their elements. 

 Thus putting sodium chloride, NaCl, and sulphuric 

 acid together, the sodium and the hydrogen of the two 

 compounds exchange places. 



2NaCl + H 2 SO 4 = Na 2 SO 4 + 2HC1 



Sodium Sulphuric Sodium Hydrochloric 



chloride acid sulphate acid 



Living matter is composed of comparatively few 

 elements. Certain of these are found in all organisms 

 while the occurrence of others is less widespread. De- 

 scriptions are here given only of the more important ones. 



