karus.] VISCOSITY OF GASES. 277 
at 500°, 1,000°, 1,500°, is not greater than 1.15, 1.22, 1.23, respectively; 
whereas its probable maximum is reached at an earlier temperature. 
Hence, if the views maintained at the present stage of molecular 
kinetics be indeed correct, then no effect of the thermal variation of 
the coefficient of external friction need in the present work be appre- 
hended. This is an important inference, for it might easily be sup- 
posed that the relation which Holman fouud to hold between 0° and 100° 
(^-=1+0.002751 1— 0.00000034 1 2 ) 
might be progressively retarded in proportion as high temperatures are 
reached, by the gradually increasing values of 
0+4) 
and in this way lead to the results which I have found. 
'Advantages of an exponential law. — It is next desirable to examine 
into the reasons in virtue of which, at the present stage of research, 
the equation ?;=?/ (l + a0")S may be accepted preferably to any other 
form. I will state here, inasmuch as one of the chief purposes of the 
present investigation is the introduction of a new instrument of high- 
temperature measurement, that any exponential form (l+a6") n which 
is in good accordance with the observations in hand is particularly 
acceptable, because it facilitates the calculation of thermal data by the 
principle of viscosity. Hence, when the choice is open between a num- 
ber of equations, apparently of equal availability, the exponential form 
will always be adopted, because of the practical advantages just stated. 
Strictly speaking, the formula accepted for if (?)"=?j (l+a6")$) is 
applicable to the case of a diatomic gas. In the case of monatomic 
gases, the supposition that the atoms are hard elastic solids leads to 
the law rj"= \Zl + «0", as Maxwell and Meyer have elaborately shown. 1 
From this law Maxwell 2 was led to depart, after having made a series 
of experiments in which viscosity appeared to vary directly as the 
absolute temperature of the gas. Maxwell thereupon deduced a law 
of repulsion between the molecules of a gas varying inversely as the 
fifth power of the distance between them, an acceptation by which 
his equations were capable of much simplification. Inasmuch as all 
the subsequent experiments made by many observers have failed to 
confirm MaxwelPs experiments, it appears from this and from other 
evidence which Meyer 3 adduces that MaxwelPs law of fifth powers 
is untenable. No other law of repulsion between molecules having 
'Maxwell: Phil. Mag. (4), vol. 19, 1860, p. 31; Meyer: Pogg. Ann., vol. 12."), 1865, 
pp. 177, 401, 564. 
2 Maxwell: Phil. Trans., I, p. 249, 1866. 
3 Cf. Maxwell: Phil. Mag. (4), vol. 35, 1868, pp. 129, 185. Mover; Kinetische The- 
orie der Gase, § 77. 
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