77 VISCOSITY OF GASES 187 



magnitude of the viscosity of rarefied air without external 

 friction coming into the question, since slip was excluded by 

 this arrangement of the experiment. From these measures 

 it resulted that Maxwell's law of the constancy of the 

 coefficient of viscosity in actual gases holds down to pressures 

 which are so small that they can no longer be measured 

 with accuracy. Only at a much higher rarefaction there 

 occurs a sudden drop in the value of the coefficient of 

 viscosity. 



Just as at very low pressures, so also at very high 

 pressures, Maxwell's law of the constancy of the vis- 

 cosity-coefficient loses its strict validity; for under these 

 circumstances, too, the assumptions of the theory no longer 

 hold good. The theory starts with the assumption that 

 a particle of gas traverses a straight length of path between 

 successive encounters with others, in comparison with which 

 the curved parts of the path that are traversed during the 

 actual encounters are vanishingly small. But this assump- 

 tion can no longer be upheld when the gas is very dense. 

 The dissociation of the molecules also may cause the law 

 to lose its admissibility, for the theory assumes that the 

 molecules are unalterable. 



That the law does not hold any longer with exactness at 

 very high or very low pressures cannot form any objection 

 against the validity of the theory under ordinary circum- 

 stances, i.e. at moderate pressures and average tempera- 

 tures ; and we may for the present leave out of account 

 that at very high pressures the value of the viscosity- 

 coefficient of a gas perceptibly increases with the density, 

 as Warburg and von Babo l have shown for carbonic 

 acid at pressures between 30 and 120 atmospheres. 



For other gases which conform more exactly to the laws 

 of perfect gases the question has not been sufficiently 

 investigated. For even if the coefficient of viscosity for air 

 and other gases seems, according to many observations, 2 to 



1 Ber. d. naturf. Ges. zu Freiburg i. B. 1882, viii. ; Wied. Ann. 1882, 

 xvii. p. 390 ; Berl. Sitzungsber. 1882, p. 509. Compare 89 and the following. 



2 Graham, Phil. Trans. 1846 and 1849 ; Pogg. Ann. 1865, cxxvii. pp. 271, 

 355, and fol. P. Hoffmann, Wied. Ann. 1884, xxi. p. 470. Barus, Bull, of 



