REVERSIBILITY
The concept of reversibility is quite simple and is most easily demonstrated through an example. Consider the ball in the figure below.
It is clear that we could apply some force to this ball and in a certain amount of time, and cause this ball to undergo the demonstrated motion. However, for a single ball there is no reason why we cannot run time backwards. Meaning that we begin at the ending point of the last situation, and end and the beginning as shown in the figure below. For a single particle there is no problem with completing this task. Thus a single particle is a reversible!
William Ostwald threw a big punch by posing the question, "How come we never see an ice cube 'unmelt'?" Meaning if the Atomists approach is to be true, and we can model matter as if it was composed of a large number of particles which are individually reversible, why is it that we never see an ice cube form at room temperature out a glass of water?
MAYBE ONE DAY WE WILL!!
Ludwig Boltzmann and the Atomists dodged this punch by showing, through a statistical analysis, that there is a STATISTICAL chance that one day we might see an ice cube 'unmelt'.
Basically what they were saying is that assuming that matter can be modeled by these particles, then there is a huge number of them in anyform of matter, this creates what is known as an ensemble of particles. In order to treat such a huge number of particles we must use statistics. From this point of view there does exist a statistical chance that an ice cube can unmelt. However, it is highly Improbable.
In order to get a better appreciation for the unlikliness of this event, consider a pile of bricks falling off of a truck as shown in the figure below.
Clearly the situation of the right is more likely to occur, however, that does not mean that the situation on the left cannot occur! The situation on the left is just highly improbable, just like the situation of an ice cube unmelting.
So the Atomists managed to dodge a huge punch thrown by the Energetics! In fact this fight lasted for 68 years until 1905 when Swiss Patent Clerk, Albert Einstein, told us the answer and changed the way we viewed matter from then on!