LIFE AND ENERGY 5 



are themselves complex organisations, and sometimes divide up 

 into other smaller elements. 



Molecules are not always composed of different kinds of atoms; 

 two or more of the same kind may be united together, as in the 

 case of those materials called chemical elements, in the free state, 

 such as the oxygen and nitrogen of the atmosphere, iron and 

 copper, and so on. 



Now, suppose that we consider how these molecules are behaving 

 in a gas such as the atmosphere. It is clear from the fact that we 

 can, by pressure, make a particular volume into a smaller one, as, 

 for example, by pushing in the piston of a syringe with the nozzle 

 closed, that the molecules cannot have been in close contact 

 originally. They must have an actual size and, therefore, there 

 must also be free spaces between them. The molecules, indeed, 

 make up a very small part of the total volume of a gas. A rough 

 idea of how little it is could be obtained by taking a flask full of 

 the vapour of water and cooling it, so that the steam is condensed to 

 water. The total number of molecules must be the same in both 

 steam and water, or, more correctly, the number of atoms must be 

 the same in both, since we shall see later that some of the molecules 

 combine together when steam condenses to water. 



Why, then, do we have to exercise pressure on a gas if we wish 

 to make its volume smaller? Why does it resist the process? It 

 is because the molecules are in a state of perpetual to-and-fro 

 movement, hitting against the vessel containing the gas with a 

 total pressure in proportion to the number of molecules that hit in 

 a given time. If we diminish the volume, we press more molecules 

 into the space than were previously there, so that we increase the 

 number of hits. Although these molecules hit against each other 

 occasionally, they are practically free from anything to hold them 

 together, so that, if a vessel containing a gas is connected to another 

 empty one, the gas divides itself equally between the two. This 

 movement of the molecules is due to their possession of that form 

 of energy which we call heat. 



In a liquid, the constituent molecules are so close together as 

 to be within the distance at which they begin to attract one 

 another. Although this attraction does not begin to be appreciable 

 until the molecules are extremely near together, it reaches a very 

 high value at that position ; so that a very great force is required 

 to pull them further apart. The attractive force between molecules 

 shows itself as cohesion, and, in the case of a liquid, is known as the 

 internal pressure of that liquid, with which we shall meet again 

 presently. The molecules of a liquid cannot, then, move further 

 apart from each other, but they can rush about with a movement 

 like that of the molecules of a gas, so long as their distance from 



