Matter and Energy. 9 



or, from a chemical point of view, the smallest quantity 

 that is able to exist in a free state, or take part in or result 

 from a chemical change. Molecules of the same kind, i.e. 

 of the same substance in whatever state it may occur, agree 

 in possessing the same weight, size, and properties, differing 

 in these respects from molecules of all other substances. 



Constitution of n.atter. Molecules by the employment 

 of certain agencies, chemical or other, are themselves capable 

 of division. Their constituent parts are termed atoms. 

 An atom may therefore be looked upon as the ultimate 

 chemical particle of matter, as distinguished from the mole- 

 cule, the ultimate physical particle. The atom, as its name 

 indicates, is itself indivisible by any means at present in 

 our power. Nearly every molecule is composed of two 

 or more atoms. In a very few cases the molecules con- 

 sist of one atom only, e.g. the metals mercury, cadmium, 

 and zinc, and in these cases the ultimate physical and 

 ultimate chemical particles may be said to be identical. 

 If all the atoms going to form a molecule be identical, then 

 the molecule, or ultimate physical particle, is an elementary 

 molecule, and a substance composed of such molecules is 

 termed a chemical element. If, however, the atoms going 

 to make up the molecule differ in size, weight, and other 

 properties, then the molecule is compound in its nature, and 

 the substance formed of such molecules is known as a 

 chemical compound. 



Molecules vary in size according to the number of atoms 

 entering into their composition that number ranging from 

 one (in a very few cases) to several hundreds. In the case 

 of elementary substances the terms monatomic, diatomic, 

 triatomic, c., are used according to the number of atoms 

 going to make up a molecule of the element in question. 



At the present time about seventy different kinds of 

 atoms, or elementary forms of matter, are known. 1 It is 



1 Since the above sentence was written Kruss and Nilson announce 

 the discovery of twenty new elements in rare Scandinavian minerals. 



