According to the usual kinetic theory of gases, the pressure and temper-ature of a gas are given by P=2n/3{mn2/2-mv2/2}, T=2/3k{mu2/2-mv2/2}, (1) where m is the mass of a gas molecule, u its Velocity, v the velocity of mass-motion, n the number of molecules per unit volume, k the gas constant per molecule, - the mean value for a number of molecules. According to the special theory of relativity, these equations do not hold true rigorously. The relativistic expressions for p and T can be obtainod by an elementary method ; they are p=2n/3{1/2(m0u2/√1-u2/c2)-1/2 μ0v2/√1-v2/c2}, T=2/3k{1/2(m0u2/√1-u2/c2)-1/2 μ0v2/√1-v2/c2}, _??_0=(m0/√1-U2/c2)=(m)0 (21) where m0 is the mass at rest of a molecule, U its velocity relative to the coosdinate system K0 in which the mass-motion of the gas is null, and 0 the miss of the gas in K0 divided by the number of the molecules. Comparing (1) and (21), we see at once that 1/2m0u2/√1-u2/c2 and 1/2 (m)0v2/√1-v2/c2 not the kinctie energy m0c2(1/√1-u2/c2 -1) and (m)_??_c2(1/√1-v2/c2 -1) in the relativistic expressions of p and T correspond to 1/2 mu2 and 1/2 mv2 in their usual expressions respectively

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