76 AN INTRODUCTION TO SCIENCE 



bles would not be in contact with one another, but sepa- 

 rated by vacant space, and not at rest but in a state of 

 perpetual motion. The intervals between the marbles (not 

 the size of the marbles) would vary according as the matter 

 was in a solid, liquid or gaseous state. They would also 

 be proportional to the amount of heat in the body. The 

 hotter the body the more rapidly its particles move ; the 

 more rapidly they move the greater are the intervals which 

 separate them, or vice versa. But if our power of vision 

 were still further increased we should see that each marble 

 is a group of smaller bodies, still not in contact, but sepa- 

 rated one from the other by very much smaller spaces than 

 those which separate the marbles. In chemical language, 

 matter is composed of molecules, and molecules of atoms. 

 The chemist attributes the properties of matter to the 

 arrangement of the atoms in its molecules. He believes 

 that when he changes the nature of a substance when 

 he alters its properties, that is to say he changes either 

 the number or the kind of atoms, or their mutual arrange- 

 ment in the molecule ; for he has the best of reasons for 

 thinking that a molecule does not consist of a certain 

 number of elementary atoms arranged at haphazard, as 

 stones of several kinds might be thrown into a sack, but 

 that the atoms are put together according to a plan so 

 definite that no two atoms could change places in a mole- 

 rule without an alteration in the properties of the sub- 

 stance. 



There are certain substances, in number about seventy, 

 which cannot be changed one into another. These are the 

 chemical " elements." Until the seventeenth century all 

 forms of matter were supposed to be transmutable. Aris- 

 totle taught that there is only one fundamental matter which 

 is united in Nature with varying quantities of the four 

 "elementary principles," earth, fire, air, water; and that 

 the properties which different forms of matter present 

 depend upon the relative amounts of the several elementary 



