Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. In the classical picture of an ideal gas, molecules bounce around at a variety of different velocities, never interacting with each other. Though this qualitative picture is obviously flawed (since molecules always do interact), it is a useful model for situations where the particle density is very low; in a more quantitative sense, this means that the particles themselves are very small when compared to the volume between them. Accordingly, we will want to know exactly how many of these molecules are moving around at a given speed. The Maxwell speed distribution (MSD), named after James Clerk Maxwell, is a probability distribution describing the spread" of these molecular speeds it is derived, and therefore only valid, assuming that we''re dealing with an ideal gas. Again, no gas is truly ideal, but our own atmosphere at STP behaves enough like the ideal situation that the MSD can be used. "