In the simple calculation of heat conduction through a gas deduced from the transport theorem, it was assumed that out of the total number of molecules contained in a cubical box, one-sixth of the molecules move in a direction perpendicular to each face of the box. In the following deduction, we shall suppose that

- the temperature of the gas increases in the direction of the
*z*axis. - the mean energy of a molecule in any layer perpendicular to the
*z*axis is that appropriate to the temperature of that layer. - the molecules moving in all possible directions have the same average velocity .

Let ...

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