OBJECTS.] JNTRODUCTORY. 69 



We are justified, in accordance with the general 

 properties of matter ( 18), in supposing that these 

 molecules tend to approach one another. But the 

 fact that water is slightly compressible justifies the 

 supposition that its molecules are not in actual con- 

 tact, but that they are separated by interspaces, just as 

 the motes in the air of a dusty room are so separated. 

 What is it that keeps the molecules apart ? We have 

 seen that great mechanical pressure brings them but 

 slightly nearer to one another ; hence there is an 

 equivalent resistance of some kind which keeps them 

 apart. This resistance must have the same origin as 

 the sensation which we know as heat, for it has been 

 seen that diminution of heat diminishes the bulk 

 of water ; that is, allows the molecules to come closer 

 together ; that is, diminishes their tendency to keep 

 asunder. Increase of heat, on the other hand, in- 

 creases the volume of water ; that is to say, drives the 

 molecules further apart, or increases their tendency 

 to keep asunder. 



Suppose we call the cause of the tendency of the 

 molecules of water to come together an attractive 

 force ; and the cause of their keeping apart, which 

 manifests itself to us as the sensation of heat and is, 

 as we have seen, in all probability, a rapid vibratory or 

 whirling motion of the molecules, a repulsive force ; 

 then, in the liquid state, these forces are so adjusted 

 that the molecules are quite free to move, and yet 

 hold together. 



By adding heat the repulsive force is increased, 

 until the molecules are about twelve times fas ar apart 

 as they were in each direction ; while the attractive 



