10 



exercise your imagination a little and to try to picture to yourself 

 what this thing is that you have been studying. You must think of 

 any portion of matter, whether it is solid, liquid or gas, as consisting 

 of an immense number of very small particles which are not at rest, 

 but all the time in vigorous motion. You can not see these particles, 

 for they are too small to be seen, but you can imagine how they act if 

 you think of people seated close together in a crowded room. If you 

 were far enough above the people you can easily understand that yoa 

 would think the whole audience to be a solid mass without motion, 

 but when you come close enough to them you can see that each one 

 has a certain amount of freedom and can move to and fro a little, 

 though always confined to a particular seat and kept pretty closely in 

 contact with his neighbors. Now, that is exactly the state of affairs 

 in a solid. The particles are continually vibrating backward and for- 

 ward, but nevertheless they do not get any considerable distance away 

 from their average position, and when you heat the solid all that 

 happens is that each particle vibrates more vigorously than before, 

 and, owing to this more vigorous motion, all the particles shove one 

 another farther apart, and so you find the solid expanding. 



But what about liquids? Think of how the same crowded audience 

 would act if they were standing up and there were no seats in the 

 way. Each one would keep bumping to and fro among his neighbors 

 and gradually, though slowly, making his way from one place to an- 

 other. This will help you to imagine what a liquid is like, for in a 

 liquid the particles are continually vibrating to and fro, frequently 

 striking against their neighbors, and thus gradually strajang from one 

 part of the vessel to another. Heating not only increases the vigor 

 of the vibration, but enables the particles to move around more freely. 

 Moreover, as in the case of the solid, we have an expansion of the 

 whole mass due to the particles shoving themselves farther apart. 



Then, as to a vapor or gas like the air, we will get an idea how the 

 particles in it behave if m'c think of skaters on a lake dashing rapidly 

 to and fro and going considerable distances without colliding with 

 others or being stopped by the banks. Here, again, a higher tem- 

 perature means a greater speed of motion of the particles. 



