1401 
I y 
log —, 
BZT ZM 
where M is the molecular weight, 5 the mean radius of a mole- 
cule, as it is effective in viscosity, and c a constant, the same 
for all substances, we may, when the curves which express the 
at 
connection between 4 and 7 are the same, infer the ratio between 
k 
the units of length which have to be ascribed to the two mechani- 
cally similar *) systems. With the help of the viscosity we can, there- 
fore, make a comparison of the just defined mean molecular radii 
and we may inquire how far the ratio found corresponds to that 
of the mean molecular radii, determined in the way that is necessary 
in the deduction of the equation of state. If this correspondence 
were complete, then, when the expression of 5 given above, is ex- 
pressed in reduced quantities, the curves which express the logarithm 
of the reduced o as a function of the logarithm of the reduced tem- 
perature for the various substances, would coincide. The accompany- 
ing diagram shows in how far this is the case. In the construction 
Mh Tile p's has been used as the ratio by which the viscosities ) 
are deduced to the same imaginary system. In this we have taken 
pre and 7% which hold for the critical state *), as determining quan- 
tities, and postponed the consideration of deviation functions still later‘). 
The- first thing that strikes one is the great deviation of helium. 
In § 1 we remarked that the character of the viscosity of helium 
can be expressed by replacing the constant c, which may be under- 
stood as a measure for the attraction between the molecules, in 
Reincanum’s formula (differing by a constant factor from v in KrersOM’s 
formula) by c/ T’log 7. Perhaps this points to an increase with the 
temperature of the quantity which determines the attraction of helium. 
= log 6 — loge 
1) More correctly : mechanically and statistically similar. 
2) H. Kamertincu Onnes Leiden Comm. No. 12, p. 9. 
3) The cristical dates we have used are the following. 
pk (Atm.) ‘Tk 
Ay 15.0 32°.0K. 
He 2.26 5 .25 
Oz 50.0 155- .0: 
Na 33.0 LO a 
Ar 48.0 150,7 
CO 35.9 132 O0 [Note added in the translation |. 
4) KAMERLINGH Onnes and Keesom. Suppl. No. 23, § 38. The ratios found by 
Keesom in Suppl. No. 25. p. 12, note 3, give 6"/) deviation for hydrogen and 
argon, those used here 9 °/o. 
