432. In the last three chapters we have considered the free path phenomena of viscosity, conduction of heat and diffusion, and have found for the three corresponding coefficients formulae involving in every case the quantity σ, the diameter of the molecule of the gas in question. Thus we have three phenomena from which the molecular diameter may be calculated.

The values of σ which can be deduced from the phenomenon of viscosity have already been calculated and exhibited in the table on p. 288. A similar set of values can be deduced from the observed values of the coefficient of conduction of heat given on p. 301. To do this, it is necessary to make some definite assumption as to the transfer of internal molecular energy, and the assumption which has been made is that already explained in § 403. Finally, it is possible to obtain a third set from the coefficients of diffusion given in the tables on pp. 323, 324, although the procedure here is rather more complicated than in the two former cases. In these tables we have thirteen observations from which to determine the eight molecular diameters involved. A leastsquare solution would be laborious, and of little real value since obviously some of the values given for D12 have a much greater observational value than others. The following simple plan has therefore been followed.

The values of the molecular diameters of the three gases hydrogen, oxygen and air have been determined solely from the first three entries in the first table. The value of σ for nitrogen can then be obtained from the fourth entry.