

§4 Maxwell's law of distribution of velocities and energies
To derive the function of the velocity distribution f(v) equal to the ratio of the number of molecules dN, the speed of which lie in the interval v ÷ v + dv to the total number of molecules N and the size of the interval dv
Maxwell used two sentences: a) all directions in space are equivalent and therefore any direction of the particle, ie, any direction of the velocity is equally likely. This property is sometimes called the property of the isotropic distribution function. b) moving along three mutually perpendicular axes that are independent, ie xcomponent of velocity v_{x} does not depend on what the meaning of its components or v_{y}, v_{z}. And then the output f(v) is initially for one component v_{x}, and then extended to all the coordinates of speed.
Using the methods of probability theory, Maxwell found the function f (v)  the distribution of ideal gas molecules in velocity:
f (v) depends on the type of gas (the mass of the molecule) and the state variable (temperature T) For small v and the function f(v) changes almost on a parabola v^{2}. With an increase in the factor decreases faster than the multiplier , ie a max function f (v). The speed at which the distribution function of molecules of an ideal gas at the maximum speed is called the most probable velocity v_{mp} from the condition:
 therefore, with increasing temperature the most probable speed increases, but the area of ??S, bounded by the curve of the distribution function remains unchanged, since the normalization condition since the probability of a certain event is 1), so that the temperature distribution curve f(v) will stretch and fall. In statistical physics, the average value of any quantity is defined as the integral from 0 to infinity, the product of the probability density for this value (statistical weight)
Then the arithmetic average velocity of the molecules
and integrating by parts received
Speed, characterizing the state the gas § 5 Experimental verification of the Maxwell distribution law – Stern experience
Along the axis of the inner cylinder tight platinum wire, covered with a layer of silver, which is heated by the current. When heated, the silver evaporates; the silver atoms are emitted through the gap and onto the inner surface of the second cylinder. If both cylinders are fixed, all atoms regardless of their speed fall in the same place B. When rotating cylinder with an angular velocity ω silver atoms get into the points B ', B'' and so on. The magnitude of ω, the distance ? and displacement x = BB 'can calculate the velocity of the atoms belonging to a point B'.
Slit image getting blurred. Exploring the thickness of the deposited layer can estimate the distribution of the velocity, which corresponds to a Maxwellian distribution.
We derive the variation of pressure with height assuming that the gravitational field is uniform, the temperature is constant and the mass of all the molecules of the same. If the atmospheric pressure at a height h equal to p, then at a height h + dh is equal to p + dp (with dh> 0, dp <0, since p decreases with increasing h). The pressure difference at the heights h and h + dh, we can determine as the weight of the air molecules enclosed in a volume with a base area equal to 1 and a height dh.
ρ  density at the height h, and since , then ρ = const. Then
Of MendeleevClapeyron equation.
Then
With the change in height from h_{1} to h_{2} pressure changes from p_{1} to p_{2}
Potentiated this expression
Barometric formula shows how the pressure changes with altitude At Then
Because the
and
n  the concentration of molecules at a height n_{0}  the concentration of molecules at a height h = 0. Because the
then
the potential energy of the molecules in a gravitational field Boltzmann distribution law in an exterior potential field. From it follows that at T = const the density of gas is more there where potential energy of molecules is less.
§ 7 The experimental determination of the Avogadro constant J. Perrin (French scientist) in 1909, studied the behavior of Brownian particles in the emulsion gamboge (tree sap) with dimensions were examined with a microscope, which had a depth of field  1 mm. Moving the microscope in the vertical direction could be to investigate the distribution of Brownian particles in height.
where m particle mass m_{1}  weight of the displaced fluid. If n_{1} and n_{2} concentration of particles at levels h_{1} and h_{2}, and
then
Value is in good agreement with the reference value, which confirms the Boltzmann distribution of particles. 