110 JAMES CLERK MAXWELL. [CHAP. V. 



geometric properties of matter. The following properties do 

 not necessarily belong to geometric figures. 



4. No part of space can contain at the same time more 

 than one body, or no two bodies can coexist in the same 

 space; this property is called impenetrability. It was 

 thought by some that the converse of this was true, and 

 that there was no part of space not filled with matter. If 

 there be a vacuum, said they, that is empty space, it must 

 be either a substance or an accident. 



If a substance it must be created or uncreated. 



If created it may be destroyed, while matter remains as 

 it was, and thus length, breadth and thickness would be 

 destroyed while the bodies remain at the same distance. 



If uncreated, we are led into impiety. 



If we say it is an accident, those who deny a vacuum 

 challenge us to define it, and say that length, breadth and 

 thickness belong exclusively to matter. 



This is not true, for they belong also to geometric 

 figures, which are forms of thought and not of matter ; there- 

 fore the atomists maintain that empty space is an accident, 

 and has not only a possible but a real existence, and that 

 there is more space empty than full. This has been well 

 stated by Lucretius. 



5. Since there is a vacuum, motion is possible ; there- 

 fore we have a fifth property of matter called mobility. 



And the impossibility of a body changing its state of 

 motion or rest without some external force is called inertia. 



Of forces acting between two particles of matter there 

 are several kinds. 



The first kind is independent of the quality of the 

 particles, and depends solely on their masses and their 

 mutual distance. Of this kind is the attraction of gravi- 

 tation and that repulsion which exists between the par- 

 ticles of matter which prevents any two from coming into 

 contact. 



The second kind depends on the quality of the particles ; 

 of this kind are the attractions of magnetism, electricity, and 



