182 PHENOMENA DEPENDENT ON MOLECULAR PATHS 77 



magnitudes calculate the amount of friction that has taken 

 place and the value of the coefficient of viscosity. The 

 external friction that also comes into play in the experi- 

 ment, that is, the friction that occurs between the fluid and 

 the surface of the disc, may be eliminated by repetition of 

 the experiments with more discs of various sizes. 



To make this method more suitable for the determina- 

 tion of the friction of the air, I altered it in my own 

 experiments 1 by employing three oscillating discs with a 

 common axis, instead of one, and arranging them either 

 apart from each other so as to put into oscillation by their 

 six faces the air above and below each of them, or all close 

 together, like a single disc, so as to move the air by two 

 faces only. The combination of the results of the two ex- 

 periments allowed the coefficient of viscosity to be calculated 

 in a simple way from the difference of the two observed 

 logarithmic decrements. Determinations of the coefficient 

 of viscosity of air carried out at different pressures by this 

 method did not, indeed, show a complete agreement with 

 each other as Maxwell's theory required, but the differ- 

 ences between the values found were small enough to prove 

 the law at least within certain limits of the pressure. As 

 an instance I cite the numbers 2 which I obtained by experi- 

 menting with an apparatus provided with three glass discs. 

 Since the method does not lead directly to a knowledge of 

 the viscosity-coefficient itself, but rather, first of all, to the 

 square root of the coefficient, I put the latter, referred to 

 centimetres, in the following table, which contains also the 

 temperature and the pressure, the former in Centigrade 

 degrees, the latter in centimetres of mercury. 



1 Am.ll. Ber. der Naturf.-Vers. in Stettin, 1863, p. 141 ; Pogg. Ann. 1865, 

 cxxv. p. 177. 



2 Calculated on an improved theory (Sitzungsber. d. Milnchencr Akad. 1887, 

 xvii. p. 343 ; Wied. Ann. 1887, xxxii. p. 642), account being taken of a correction 

 suggested by W. Konig (ibid. 1887, xxxii. p. 193). 



