NATURE OF LIFE AND LIVING MATERIAL 13 



for some few water molecules, (HOH), dissociate into H and OH 

 ions, but in pure water their numbers are exactly equal. 



Salts. When the hydrogen atoms of an acid are replaced by 

 some other type of atom, the compound is known as a salt. Thus 

 if the H atom in hydrogen chloride (hydrochloric acid, HCl) is 

 replaced by a sodium atom, the compound formed is common table 

 salt, NaCl. If the two hydrogen atoms in sulphuric acid, H-SO4, 

 are replaced by a single magnesium atom, Mg, the resulting com- 

 pound is magnesium sulphate, MgS04, commonly known as Epsom 

 Salts. Salts of inorganic acids ionize freely when in solution and 

 thus exhibit electrical properties as do acids and bases. 



Compounds in Living Matter. The elements found in 

 protoplasm are comparatively few, twelve or more of the common 

 ones, oxygen, hydrogen, nitrogen, sodium, iron, potassium, calcium, 

 phosphorus, sulphur, chlorine, iodine, and usually traces of others. 

 It is worth noting that none of the rare elements are found; so the 

 behavior of protoplasm cannot be ascribed to any peculiar element. 

 Living protoplasm contains inorganic salts, bases, and acids. These 

 are dissociated, that is, ionized, to some extent in protoplasm, for 

 protoplasm contains a large proportion of water. If the number of 

 molecules of one of these compounds is changed, changes also occur 

 in the others because the electrical conditions are disturbed. Par- 

 ticularly is this true of the balance between acid and alkali; it ap- 

 pears that this balance is essential in the life of the material, for 

 any great disturbance of the acid-alkali balance constitutes a serious 

 injury to the living mechanism. Of the inorganic molecules in proto- 

 plasm, undoubtedly water is the most prominent and in many re- 

 spects the most important. Its usefulness will be referred to farther 

 on. Water almost invariably forms at least 70 per cent of the weight 

 of living bodies. 



The chief organic compounds in protoplasm may be grouped into 

 three categories. Of these proteins are the most prominent. These 

 are complex chain molecules with many carbon atoms linked to- 



